Plural nouns name more than one person, animal, thing, or idea. English usually marks plural number by adding s or es to a regular noun, but many common nouns form their plural in less predictable ways. Irregular plural forms still function as normal plural nouns, so they follow ordinary agreement patterns in sentences and combine with the same noun system described in Nouns.

Regular plurals are formed by adding s or es, while irregular plurals change in ways that cannot be predicted from the singular spelling alone. The contrast matters because the plural meaning is clear in both forms, but the spelling of irregular nouns must be learned individually. Learners also meet these forms when studying Counting Nouns, where number is especially important.

Idea	Example
Regular nouns usually add s or es to show plural number.	cat becomes cats
Irregular nouns may change the word inside instead of adding a visible ending.	child becomes children
Some irregular plurals must be memorized because the pattern is not predictable.	man becomes men

Several common irregular plurals are formed by changing the vowel inside the word. This pattern appears in a small group of high frequency nouns such as man, foot, and mouse, which become men, feet, and mice. These plurals keep the same basic meaning but show number through internal vowel change rather than an added ending.

Idea	Example
A changed vowel can mark plural meaning in some common nouns.	man becomes men
A fronted vowel can mark plural meaning in some common nouns.	foot becomes feet
A different internal vowel can mark plural meaning in some common nouns.	mouse becomes mice

Some nouns ending in f or fe change to ves in the plural. This pattern appears in words such as leaf, life, and knife, but it is not fully automatic because several common nouns simply add s, such as roof becoming roofs. The spelling therefore depends on the individual word rather than the ending alone.

Idea	Example
Some nouns change f to ves in the plural.	leaf becomes leaves
Some nouns change fe to ves in the plural.	life becomes lives
Some nouns keep f and add s instead of changing form.	roof becomes roofs

Some English nouns preserve plural patterns from Latin or Greek. Words ending in us may become i, as in cactus and cacti, while words ending in is often become es, as in analysis and analyses. Other learned forms include on or um changing to a, as in phenomenon and phenomena or datum and data, and many of these nouns also have accepted Englishized alternatives.

Idea	Example
Some Latin based nouns ending in us take i in the plural.	cactus becomes cacti
Some Greek or Latin based nouns ending in is take es in the plural.	analysis becomes analyses
Some learned nouns ending in on or um take a in the plural.	phenomenon becomes phenomena

Some nouns have the same form in both singular and plural. This group includes sheep, deer, and fish, although fish can also take fishes when different species are being discussed. The plural meaning is therefore shown by the surrounding words rather than by a visible noun ending, which is especially useful in Collective Nouns.

Idea	Example
Some nouns keep the same form in singular and plural.	one sheep and many sheep
Some animal nouns are unchanged in ordinary plural use.	one deer and many deer
Fish can be unchanged, or fishes can refer to species.	fish in the river and fishes in the book

A number of plural nouns come from other languages and keep a foreign sounding ending in educated or formal usage. Examples include criterion becoming criteria and appendix becoming appendices, although Englishized forms such as criterions or indexes are also accepted in some contexts. These forms are regular in meaning even when their plural spelling reflects older language patterns.

Idea	Example
Some nouns borrowed from Latin form plural with ia or a sound changes.	criterion becomes criteria
Some nouns borrowed from Latin or Greek form plural with ices or es endings.	appendix becomes appendices
Some borrowed nouns have an Englishized plural form beside the classical form.	index becomes indexes

Some nouns are normally used only in plural form and take plural agreement. Scissors, trousers, and glasses are common examples, and they behave as plural nouns even when they refer to one object made of two connected parts. Their plural meaning is built into the noun form, so singular verbs and singular determiners are not used with them. Learners meet similar agreement patterns when studying Proper Nouns in fixed names and titles.

Idea	Example
Some nouns are always treated as plural in grammar.	the scissors are sharp
Some clothing nouns have plural agreement even when one item is meant.	the trousers are new
Some glasses nouns are plural only in form.	my glasses are on the table

In compound nouns, the main noun usually takes the plural ending. Mothers in law becomes mothers in law, and passer by becomes passers by, because the head noun carries the number meaning. This pattern shows that plural marking follows the structure of the whole noun phrase rather than the nearest word alone.

Idea	Example
The head noun in a compound usually takes the plural ending.	mother in law becomes mothers in law
A compound made with passer by pluralizes the main noun.	passer by becomes passers by
Plural marking follows the structure of the compound noun.	sister in law becomes sisters in law

Irregular plurals still require plural determiners and plural verbs. These children are happy, those men were late, and many deer have crossed the field all show that the noun phrase agrees in number with the plural noun form. The same agreement principle supports clear number marking in Collective Nouns, although some collective nouns vary by region.

Idea	Example
Irregular plural nouns take plural determiners.	these children are laughing
Irregular plural nouns take plural verbs.	those men were ready
Plural agreement is required even when the plural form is irregular.	many feet are cold

Plural nouns form possession by adding apostrophe s after the plural form when needed. Children becomes children's, men becomes men's, and mothers in law becomes mothers in law's, because the possessive ending is attached to the already plural noun. The same rule applies to irregular and regular plurals alike, with the only difference being the plural form that comes before the possessive ending.

Idea	Example
Plural nouns form the possessive after the plural ending.	children's books are on the shelf
Irregular plurals also take apostrophe s for possession.	men's shoes are by the door
Compound plurals take the possessive on the plural head noun.	mothers in law's advice was helpful

Some irregular plurals allow more than one accepted form, and usage can vary by region, style, or level of formality. Octopuses and octopi are both heard, but the Englishized form is often preferred in general use, while team is or team are depends on whether the speaker treats the collective noun as a unit or as a group of individuals. Such variation belongs to ordinary English usage and should be recognized alongside the standard patterns.

Region	Word or Phrase	Regional Definition	Example
General English	octopuses	The Englishized plural is widely accepted in modern usage.	The aquarium has octopuses, and they are calm
Classical style	octopi	The Latin styled plural is used by some speakers and writers.	The menu lists octopi, and they are grilled
US and UK	team agreement	Singular or plural agreement may be chosen depending on whether the group is seen as one unit or many members.	The team is winning, and the players are excited

Irregular plural nouns are part of the normal noun system, so they must be recognized, formed, and used with correct agreement in context. Some change by vowel shift, some change by f to ves, some preserve classical endings, some stay unchanged, and some exist only in plural form. Once the pattern is known, the plural noun behaves like any other plural noun in determiners, verbs, and possession, which completes the number system introduced at the start and connects naturally to Counting Nouns.