Irregular Plural Nouns in English

Most countable nouns in English form their plural by adding -s or -es. For example, book becomes books, teacher becomes teachers, and box becomes boxes. However, some nouns do not follow these usual plural rules.

These nouns are commonly called irregular plural nouns.

Instead of simply adding -s or -es, an irregular plural may change a vowel, take a special ending, change considerably, or remain exactly the same as the singular form.

For example:

• man → men
• woman → women
• child → children
• tooth → teeth
• mouse → mice
• person → people
• sheep → sheep

English also contains borrowed nouns with special plural forms, such as criterion → criteria and analysis → analyses, as well as nouns that allow more than one plural form in different contexts.

Because there is no single rule for every irregular plural noun, learners need to understand the main patterns, recognize important exceptions, and learn common singular and plural forms together.

What Are Irregular Plural Nouns?

An irregular plural noun is a noun whose plural does not follow the ordinary pattern of simply adding -s or -es to the singular form.

Compare:

For example:

• one man → two men
• one child → three children
• one mouse → several mice
• one sheep → five sheep

Incorrect:

• ✗ mans
• ✗ childs
• ✗ mouses
• ✗ sheeps

Correct:

• ✓ men
• ✓ children
• ✓ mice
• ✓ sheep

The irregular form itself expresses plural meaning, so another plural -s is not normally added.

Regular vs Irregular Plural Nouns

The main difference between regular and irregular plural nouns is how the plural is formed.

Regular Plural Nouns

Regular nouns follow normal plural-forming patterns.

• cat → cats
• book → books
• bus → buses
• box → boxes

Some regular plural spelling rules involve a spelling change:

• baby → babies
• wife → wives
• potato → potatoes

These forms follow recognizable spelling patterns, even though they do not all simply add -s.

Irregular Plural Nouns

Irregular plurals have forms that cannot be produced reliably by applying the ordinary plural rules.

• man → men
• child → children
• mouse → mice
• person → people
• sheep → sheep

Compare:

This distinction is important because not every plural with a spelling change is a truly irregular plural.

Why Are Some Plural Nouns Irregular?

English has developed over many centuries and has been influenced by different languages.

Some irregular plurals preserve older English plural patterns:

• man → men
• foot → feet
• child → children
• ox → oxen

Other special plural forms entered English through words borrowed from languages such as Latin and Greek:

• criterion → criteria
• phenomenon → phenomena
• analysis → analyses

This is why modern English does not have one rule that explains every irregular plural.

Learners do not need to study the complete history of these words. What matters is recognizing the main modern patterns and learning forms that cannot be predicted reliably.

Types of Irregular Plural Nouns

Irregular plural nouns can be organized into several useful groups.

Internal Vowel Change

The vowel inside the noun changes:

• man → men
• foot → feet
• mouse → mice

Special Ending

The noun takes an unusual plural ending:

• child → children
• ox → oxen

Substantial Change

The plural changes considerably:

• person → people

No Change

The singular and plural forms are identical:

• sheep → sheep
• deer → deer
• species → species

Foreign or Classical Plural

A borrowed noun retains or uses a special plural form:

• analysis → analyses
• criterion → criteria
• phenomenon → phenomena

These groups make irregular plurals easier to learn than one long, unorganized list.

Irregular Plurals with Internal Vowel Changes

Several common English nouns form their plurals by changing a vowel inside the word.

Examples:

• One man was waiting outside.
• Three men were waiting outside.
• My foot hurts.
• Both my feet hurt.
• The baby has one tooth.
• The child has several teeth.
• We saw a mouse in the kitchen.
• We saw two mice in the kitchen.

Do not add the regular plural ending:

• ✗ mans → ✓ men
• ✗ foots → ✓ feet
• ✗ tooths → ✓ teeth
• ✗ mouses → ✓ mice

These forms should be learned as singular–plural pairs.

Irregular Plurals with Special Endings

A small number of English nouns form their plurals with unusual endings instead of the regular -s or -es endings.

Child → Children

The plural of child is children.

• one child
• two children
• many children

Examples:

• The child is playing.
• The children are playing.

Do not add another plural ending:

• ✗ childs
• ✗ childrens
• ✓ children

Ox → Oxen

The plural of ox is oxen.

• one ox
• two oxen

Example:

• Two oxen were pulling the cart.

The ending -en is not a productive plural rule in modern English. Learners should therefore learn ox → oxen as an individual pair rather than adding -en to other nouns.

Irregular Plurals That Change Substantially

Some singular and plural forms are quite different from each other.

The most important example for everyday English is:

person → people

Examples:

• One person is waiting.
• Five people are waiting.

This form cannot be predicted from an ordinary spelling rule, so it needs to be learned individually.

Person, People, Persons and Peoples

The forms person, people, persons, and peoples are related, but they are not interchangeable.

Person → People

In ordinary English, people is the normal plural of person.

• one person
• two people
• many people

Examples:

• One person entered the room.
• Several people entered the room.

For normal everyday use:

person → people

Persons

Persons is also a possible plural of person, especially in formal, legal, official, or administrative contexts where separate individuals are being counted.

For example:

• The lift has a maximum capacity of eight persons.

In everyday English, people is normally the natural choice.

A People → Peoples

People can also mean a particular nation, ethnic group, or community. In this meaning, it can be countable.

• a people
• different peoples

For example:

• The region is home to several indigenous peoples.

Compare:

• people = more than one person
• a people = a nation or community
• peoples = different nations or communities

Irregular Plurals That Do Not Change

Some nouns have exactly the same form in the singular and plural.

These are often called unchanged plurals or zero plurals.

Common examples include:

Examples:

• one sheep → ten sheep
• one deer → four deer
• one species → several species
• one aircraft → three aircraft

Do not add an unnecessary -s:

• ✗ sheeps
• ✓ sheep
• ✗ deers
• ✓ deer
• ✗ aircrafts
• ✓ aircraft

How to Tell Singular and Plural Apart

Because the noun itself does not change, look at the surrounding words.

Compare:

• This sheep is young.
• These sheep are young.
• One species is endangered.
• Several species are endangered.
• That aircraft has landed.
• Those aircraft have landed.

Determiners, numbers, quantifiers, and verbs help show whether the noun is singular or plural.

Animal Nouns with Unchanged or Variable Plurals

Animal names make up an important group of nouns whose singular and plural forms can be identical.

Examples include:

• sheep → sheep
• deer → deer
• fish → fish
• bison → bison
• elk → elk
• moose → moose

Examples:

• We saw three deer.
• Several bison were grazing.
• Two moose crossed the road.

However, some animal nouns have alternative plural forms or special usage. Fish is the most important example for learners.

Fish vs Fishes

Both fish and fishes exist, but they are normally used differently.

Fish

Fish is the usual plural when referring to individual fish.

• one fish
• two fish
• many fish

Example:

• We caught five fish.

Fishes

Fishes is used especially when referring to different species or kinds of fish, particularly in scientific contexts.

Example:

• The reef is home to many tropical fishes.

For ordinary English, learners should normally use:

one fish → two fish

Irregular Plurals with More Than One Accepted Form

Some English nouns have more than one accepted plural.

Examples include:

The alternatives are not always used in exactly the same contexts.

For some words:

• one form may be more common in everyday English;
• another may be more common in academic or technical English;
• different plural forms may develop different meanings.

Therefore, learners should not assume that two accepted plurals are always completely interchangeable.

When usage is uncertain, check the individual noun in a current learner’s dictionary.

Foreign and Classical Irregular Plurals

English contains many nouns borrowed from Latin and Greek. Some retain traditional plural patterns.

These forms are particularly common in academic, scientific, medical, and formal English.

Nouns Ending in -is: -is → -es

Some nouns ending in -is form their plurals by changing -is to -es.

Examples:

• one analysis → several analyses
• one crisis → several crises
• one hypothesis → two hypotheses

The pronunciation changes as well as the spelling.

Nouns Ending in -on: -on → -a

Some Greek-derived nouns ending in -on have traditional plurals ending in -a.

Examples:

• one criterion → several criteria
• one phenomenon → several phenomena

Common mistakes:

• ✗ a criteria
• ✓ a criterion
• ✗ one phenomena
• ✓ one phenomenon

Nouns Ending in -um: -um → -a

Some Latin-derived nouns ending in -um traditionally form their plurals in -a.

Examples:

• bacterium → bacteria
• curriculum → curricula
• datum → data
• medium → media

However, modern usage varies from word to word.

For example, media has developed broader uses in modern English, while mediums is used for some meanings of medium.

This is why classical plural patterns should not be applied mechanically.

Nouns Ending in -us: -us → -i

Some Latin-derived nouns ending in -us traditionally have plurals in -i.

Examples:

• alumnus → alumni
• stimulus → stimuli
• cactus → cacti
• fungus → fungi

But not every noun ending in -us follows this pattern, and some nouns also accept an English plural.

For example:

• cactus → cacti / cactuses

Do not assume:

every -us noun → -i

Learn individual forms instead.

Nouns Ending in -a: -a → -ae

Some classical nouns ending in -a traditionally form their plurals in -ae.

Examples include:

• formula → formulae
• vertebra → vertebrae

Some also have regular English alternatives:

• formula → formulas / formulae

Again, actual modern usage depends on the individual word and context.

Nouns Ending in -ex or -ix: → -ices

Some classical nouns ending in -ex or -ix have traditional plurals ending in -ices.

Examples:

• index → indices
• appendix → appendices
• matrix → matrices

Some also have regularized English plurals:

• index → indexes
• appendix → appendixes

Different forms can sometimes be preferred in different meanings or fields.

Classical vs Modern English Plurals

Classical plural patterns are useful, but they are not universal rules of modern English.

Some borrowed nouns preserve their traditional plural:

• criterion → criteria
• phenomenon → phenomena

Some allow both traditional and regular English forms:

• cactus → cacti / cactuses
• formula → formulae / formulas
• index → indices / indexes

Other foreign words simply take an ordinary English plural.

Therefore, do not create a plural only by looking at the final letters of a borrowed noun.

When in doubt, check a current dictionary.

Irregular Plurals in Compound Nouns

Compound nouns contain two or more elements that function together as one noun.

The plural normally appears on the main noun in the compound.

Compounds Ending in an Irregular Noun

When the main noun has an irregular plural, that irregular form is retained.

Examples:

• snowman → snowmen
• policeman → policemen
• policewoman → policewomen
• businessman → businessmen
• businesswoman → businesswomen

Examples:

• One policeman arrived.
• Three policemen arrived.

Compounds with the Main Noun Earlier

In some compound nouns, particularly compounds containing prepositions, the main noun comes before the end.

Examples:

• mother-in-law → mothers-in-law
• father-in-law → fathers-in-law
• sister-in-law → sisters-in-law
• brother-in-law → brothers-in-law
• passer-by → passers-by

Incorrect:

• ✗ mother-in-laws
• ✓ mothers-in-law

The important principle is to identify the main noun rather than automatically pluralizing the final word.

Man and Woman in Compound Nouns

Some compounds containing man or woman preserve their irregular plural changes.

Examples:

• businessman → businessmen
• businesswoman → businesswomen
• policeman → policemen
• policewoman → policewomen

Some open combinations can also change man/woman when plural:

• woman doctor → women doctors

Because compound structures vary, learners should check unfamiliar forms rather than assuming that every compound behaves identically.

Irregular Plurals vs Special Plural Spelling Patterns

Learner resources often use irregular plurals broadly to include forms such as wife → wives and potato → potatoes.

However, it is useful to distinguish these predictable or semi-predictable spelling patterns from strongly irregular forms such as:

• man → men
• child → children
• person → people

The following patterns are related to plural formation but should not be confused with completely irregular forms.

Nouns Ending in -f or -fe

Some nouns ending in -f or -fe change to -ves.

Examples:

• wife → wives
• knife → knives
• life → lives
• leaf → leaves
• loaf → loaves
• wolf → wolves
• shelf → shelves

However, other nouns simply add -s:

• roof → roofs
• chief → chiefs
• belief → beliefs
• chef → chefs
• cliff → cliffs

Compare:

Do not automatically change every final f to v.

Nouns Ending in -o

Some nouns ending in -o take -es:

• potato → potatoes
• tomato → tomatoes
• hero → heroes
• echo → echoes

Others take -s:

• photo → photos
• piano → pianos
• radio → radios
• video → videos

Because similar-looking -o nouns can behave differently, learners should check unfamiliar forms.

Irregular Plural Nouns vs Plural-Only Nouns

Irregular plurals should not be confused with plural-only nouns.

An irregular plural has a related singular form:

• child → children
• man → men
• mouse → mice

A plural-only noun is normally used only in a plural form.

Examples include:

• scissors
• trousers
• jeans
• shorts
• binoculars
• glasses
• clothes

Compare:

Irregular plural

• one child
• two children

Plural-only noun

• a pair of scissors
• two pairs of scissors

Plural-only nouns such as scissors and trousers normally take plural verbs:

• These scissors are sharp.
• My trousers are new.

When referring to one item consisting of two parts, we often use a pair of:

• a pair of scissors
• a pair of trousers
• a pair of glasses

Irregular Plural Nouns vs Uncountable Nouns

An unchanged irregular plural is also different from an uncountable noun.

Compare:

sheep

• one sheep
• two sheep
• five sheep

The noun is countable even though its plural form does not change.

information

• some information
• a lot of information

We do not normally say:

• ✗ two informations

So:

sheep → sheep = countable noun with an unchanged plural

information = normally uncountable; it does not have an ordinary plural in its uncountable meaning

Other common uncountable nouns include:

• advice
• furniture
• equipment
• luggage
• knowledge

A noun having no visible -s plural does not automatically make it an irregular plural noun.

How Irregular Plural Nouns Are Used in Sentences

Irregular plural nouns behave like other plural count nouns once the correct plural form has been chosen.

Their plural meaning affects articles, determiners, numbers, quantifiers, and verb agreement.

Articles and Determiners with Irregular Plurals

A singular countable noun can normally be used with a/an, but a plural noun cannot.

Compare:

• a child → children
• a man → men
• a woman → women
• a mouse → mice

Correct:

• a child
• three children
• the children
• some children

Incorrect:

• ✗ a children

Demonstratives also change according to number:

• this child → these children
• that man → those men
• this mouse → these mice
• that person → those people

Examples:

• This child is tired.
• These children are tired.
• That person was helpful.
• Those people were helpful.

Numbers and Quantifiers with Irregular Plurals

Irregular plural count nouns can be used with numbers:

• two men
• three women
• four children
• five mice
• six sheep

They can also be used with plural quantity words:

• many children
• several people
• a few mice
• many women
• several criteria
• a few sheep

The unusual form of the noun does not change its status as a plural count noun.

Subject–Verb Agreement with Irregular Plural Nouns

Irregular plural nouns normally take plural verbs.

Compare:

• The child is sleeping.
• The children are sleeping.
• The man works here.
• The men work here.
• The mouse was hiding.
• The mice were hiding.

This is also true when the singular and plural noun forms look identical:

• One sheep is missing.
• Three sheep are missing.
• This species is endangered.
• These species are endangered.

Do not decide verb agreement from the appearance of the noun alone. Determine whether its meaning is singular or plural.

Possessive Forms of Irregular Plural Nouns

Plural formation and possession are different.

When an irregular plural does not end in -s, add ‘s to form the possessive.

Examples:

• children → children’s
• men → men’s
• women → women’s
• people → people’s
• mice → mice’s
• geese → geese’s

Examples in phrases:

• the children’s toys
• the men’s changing room
• women’s clothing
• people’s opinions
• the mice’s food

Compare with a regular plural ending in -s:

• students → students’
• teachers → teachers’

Incorrect:

• ✗ childrens’ toys

Correct:

• ✓ children’s toys

Children is already plural. The ‘s shows possession.

Pronunciation of Common Irregular Plurals

Irregular plural formation can involve pronunciation as well as spelling.

For example:

• man → men
• woman → women
• foot → feet
• tooth → teeth
• mouse → mice
• goose → geese

The plural women is particularly important because its pronunciation changes more than the spelling may suggest.

Borrowed plurals can also change pronunciation:

• analysis → analyses
• crisis → crises
• thesis → theses

Learners should therefore learn important irregular plurals as spoken and written forms, not only as spelling pairs.

How to Identify an Irregular Plural Noun

There is no single ending that identifies every irregular plural. Use the following process.

1. Find the noun.

Example:

The children are playing outside.

The noun is children.

2. Decide whether the noun is singular or plural.

Children refers to more than one child, so it is plural.

3. Find the singular form.

children → child

4. Compare the singular and plural forms.

The noun does not form its plural as childs. It changes to children.

5. Identify the irregular pattern.

child → children uses a special irregular ending.

Another example:

Three sheep are standing near the gate.

• noun → sheep
• meaning → plural
• singular → sheep
• plural → sheep
• pattern → unchanged plural

Context is especially important when the singular and plural forms are identical.

How to Learn and Remember Irregular Plural Nouns

Irregular plural nouns become easier when they are learned in organized groups.

Learn singular and plural together.

Instead of learning only mice, learn:

mouse → mice

Instead of learning only children, learn:

child → children

Group similar patterns.

Vowel change:

• man → men
• foot → feet
• tooth → teeth
• mouse → mice

Special ending:

• child → children
• ox → oxen

No change:

• sheep → sheep
• deer → deer
• species → species

Classical forms:

• analysis → analyses
• criterion → criteria
• phenomenon → phenomena

Use the forms in sentences.

Do not memorize only:

child → children

Also practise:

• One child is outside.
• Three children are outside.

This connects plural formation with actual grammar.

Notice exceptions rather than inventing rules.

For example:

• leaf → leaves
• roof → roofs

Do not assume that every similar-looking word follows the same pattern.

Check unfamiliar plurals.

A reliable learner’s dictionary is especially useful for:

• borrowed nouns;
• nouns with two accepted plurals;
• compound nouns;
• uncommon irregular nouns.

Common Irregular Plural Nouns by Pattern

Vowel Change

Special or Substantial Change

No Change

Common Classical Forms

Common Mistakes with Irregular Plural Nouns

Adding -s to a noun that already has an irregular plural

• ✗ childrens
• ✓ children
• ✗ mens
• ✓ men
• ✗ mices
• ✓ mice

Using a regular plural instead of the irregular form

• ✗ mans
• ✓ men
• ✗ tooths
• ✓ teeth
• ✗ mouses
• ✓ mice

Adding -s to an unchanged plural

• ✗ sheeps
• ✓ sheep
• ✗ deers
• ✓ deer

Using a plural form as singular

• ✗ a criteria
• ✓ a criterion
• ✗ one phenomena
• ✓ one phenomenon

Using singular verb agreement with a plural noun

• ✗ The children is playing.
• ✓ The children are playing.
• ✗ The people was waiting.
• ✓ The people were waiting.

Forgetting plural agreement with an unchanged noun

• ✗ Three sheep is outside.
• ✓ Three sheep are outside.

Using the wrong demonstrative

• ✗ this children
• ✓ these children
• ✗ that people
• ✓ those people

Pluralizing the wrong part of a compound noun

• ✗ mother-in-laws
• ✓ mothers-in-law
• ✗ passer-bys
• ✓ passers-by

Applying -ves to every noun ending in -f

• ✗ rooves
• ✓ roofs
• ✗ cheves
• ✓ chiefs

Confusing irregular plurals with uncountable nouns

• ✗ two informations
• ✓ two pieces of information

But:

• ✓ two sheep

Adding an apostrophe to make a plural

• ✗ three child’s
• ✓ three children

An apostrophe does not normally form an ordinary plural.

Forming an irregular plural possessive incorrectly

• ✗ childrens’ books
• ✓ children’s books

Irregular Plural Nouns Practice

Write the correct plural form.

1. man → ______
2. woman → ______
3. child → ______
4. foot → ______
5. tooth → ______
6. mouse → ______
7. goose → ______
8. person → ______
9. sheep → ______
10. deer → ______
11. analysis → ______
12. criterion → ______
13. phenomenon → ______
14. ox → ______
15. mother-in-law → ______

Choose the correct answer.

16. Three man / men were waiting outside.
17. The childs / children are playing.
18. We saw six sheep / sheeps in the field.
19. These woman / women work together.
20. Several mouse / mice were hiding behind the cupboard.
21. We considered several criterion / criteria.
22. Scientists observed several phenomenon / phenomena.
23. Two mother-in-laws / mothers-in-law attended the meeting.
24. The people is / people are waiting.
25. Three deer was / deer were standing near the road.
26. I bought two knifes / knives.
27. The building has two roofs / rooves.

Choose the correct singular or plural form.

28. One important ______ was missing from the report.
    criterion / criteria
29. Several unusual ______ were observed.
    phenomenon / phenomena
30. This ______ is endangered.
    species / specieses
31. These ______ are endangered.
    species / specieses

Correct the mistakes.

32. The childrens are playing outside.
33. I saw three deers in the forest.
34. That people are waiting for us.
35. One criteria is especially important.
36. The womans are working upstairs.
37. My two sister-in-laws live nearby.
38. These sheeps are very young.
39. The mouses were under the table.
40. Three person were waiting outside.
41. The childrens’ books are on the desk.

Answers

1. men
2. women
3. children
4. feet
5. teeth
6. mice
7. geese
8. people
9. sheep
10. deer
11. analyses
12. criteria
13. phenomena
14. oxen
15. mothers-in-law
16. men
17. children
18. sheep
19. women
20. mice
21. criteria
22. phenomena
23. mothers-in-law
24. people are
25. deer were
26. knives
27. roofs
28. criterion
29. phenomena
30. species
31. species
32. The children are playing outside.
33. I saw three deer in the forest.
34. Those people are waiting for us.
35. One criterion is especially important.
36. The women are working upstairs.
37. My two sisters-in-law live nearby.
38. These sheep are very young.
39. The mice were under the table.
40. Three people were waiting outside.
41. The children’s books are on the desk.

FAQs

What is an irregular plural noun?

An irregular plural noun is a noun that does not form its plural by simply adding -s or -es. Instead, the word may change internally, take a special ending, or remain unchanged. Examples include man → men, child → children, mouse → mice, and sheep → sheep.

What are some common irregular plural nouns?

Common irregular plural nouns include man → men, woman → women, child → children, person → people, foot → feet, tooth → teeth, mouse → mice, goose → geese, sheep → sheep, deer → deer, and fish → fish.

What are the main types of irregular plural nouns?

The main types include nouns that change an internal vowel, such as foot → feet; nouns with special plural forms, such as child → children; nouns that stay the same, such as sheep → sheep; and borrowed or classical plurals, such as criterion → criteria and phenomenon → phenomena.

Is children already plural?

Yes. Children is already the plural form of child, so you should not add -s to it.

• ✓ one child → two children
• ✗ two childrens

Is people the plural of person?

Yes. In everyday English, people is the normal plural of person.

• one person
• two people
• many people

Persons is also correct in certain formal, legal, and official contexts.

Is fish or fishes the correct plural of fish?

Both fish and fishes are correct, but they are used differently. Fish is the usual plural when referring to individual fish, while fishes is mainly used for different species or kinds of fish.

• We caught five fish.
• The reef contains many species of fishes.

Final Takeaway

Irregular plural nouns do not form their plurals simply by following the ordinary -s/-es pattern.

Their plural forms may involve:

• an internal vowel change: man → men
• a special ending: child → children
• a substantial change: person → people
• no visible change: sheep → sheep
• a classical or borrowed form: criterion → criteria
• irregularity within a compound: snowman → snowmen

Some nouns also have more than one accepted plural, while related forms such as wife → wives follow special spelling patterns rather than the same kind of irregularity as child → children.

To use irregular plurals correctly:

1. learn common singular–plural pairs;
2. group similar forms by pattern;
3. distinguish irregular plurals from plural-only and uncountable nouns;
4. use plural determiners, quantifiers, and verbs with plural forms;
5. learn alternative and classical plurals individually where necessary;
6. check a reliable dictionary when a plural is uncertain.

Understanding the patterns is useful, but irregular plurals cannot all be predicted from one rule. Learning the most common forms in context is the most reliable way to use them accurately.

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