Some English nouns form their plurals irregularly. They do not simply add s or es, and their plural forms must be learned as fixed patterns. These plurals often come from older forms of English or from borrowed words. Memorizing the common patterns makes them predictable within groups.

Rule
Some nouns change internal vowels to form the plural.
Some nouns keep the same form for singular and plural.
Some nouns end in f or fe and change to ves in the plural.
Some nouns from Latin or Greek keep classical plural endings.

Several high-frequency nouns form the plural by changing a vowel inside the word. These pairs are treated as distinct forms, not as regular spelling changes. The singular and plural can differ in both spelling and pronunciation. This pattern is stable for the most common words in this group.

Word/Phrase	Definition
man	The plural is men.
woman	The plural is women.
foot	The plural is feet.
tooth	The plural is teeth.
goose	The plural is geese.
mouse	The plural is mice.

Some nouns have the same form for singular and plural. In these cases, the noun does not add s or es and does not change internally. The number is shown by the determiner or by context, not by the noun ending. This group includes many animal words and some measure words.

Word/Phrase	Definition
sheep	The plural is sheep.
fish	The plural is fish.
deer	The plural is deer.
species	The plural is species.
aircraft	The plural is aircraft.

Several nouns ending in f or fe form the plural by changing the ending to ves. This is a spelling and pronunciation change that applies to specific words, not to all nouns with f or fe. Other nouns with f or ff usually take regular s, so the pattern must be learned by word. The most common ves plurals are stable in standard usage.

Word/Phrase	Definition
leaf	The plural is leaves.
wolf	The plural is wolves.
knife	The plural is knives.
life	The plural is lives.
wife	The plural is wives.

Some nouns from Latin or Greek keep classical plural endings in English. The most common endings are us to i, is to es, on to a, and um to a. These plurals are typical in academic or scientific language but also appear in everyday vocabulary. For many words, English may also allow a regular s plural as an alternative.

Word/Phrase	Definition
child	The plural is children.
person	The plural is people.
cactus	The plural is cacti.
focus	The plural is foci.
analysis	The plural is analyses.
criterion	The plural is criteria.
phenomenon	The plural is phenomena.

Irregular plurals may change pronunciation as well as spelling. Some pairs have a vowel change that affects the spoken form, not just the written form. Some words have stress shifts between singular and plural, especially in borrowed terms. Clear pronunciation helps distinguish pairs in speech.

Word	Notation	Description
woman	ˈwʊm.ən	The singular has the vowel ʊ in the first syllable.
women	ˈwɪm.ɪn	The plural has the vowel ɪ in both syllables.
tooth	tuːθ	The singular has the vowel uː.
teeth	tiːθ	The plural has the vowel iː.
mouse	maʊs	The singular has the diphthong aʊ.
mice	maɪs	The plural has the diphthong aɪ.

Some irregular plurals create common spelling pairs that must be kept distinct. These contrasts often appear in writing errors, especially with e, ee, and i changes. Treat each pair as a fixed mapping from singular to plural. Regular review of high-frequency pairs supports accuracy.

Word/Phrase	Definition
man/men	Man is singular; men is plural.
goose/geese	Goose is singular; geese is plural.
tooth/teeth	Tooth is singular; teeth is plural.
mouse/mice	Mouse is singular; mice is plural.
child/children	Child is singular; children is plural.

Irregular plurals in English follow a small set of common patterns: vowel change, no change, f to ves, and classical endings. The highest-frequency words in each group are best learned as pairs. Accurate use depends on both spelling and pronunciation. Recognizing these patterns makes irregular plurals more predictable.