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Irregular plurals refer to the plural forms of nouns that do not form their plural in the regular way. Most nouns in English add -s to the singular form to make the plural form, as in boy to boys.

Some add -es to the singular form to make the plural as in church to churches. Nouns ending in a consonant followed by -y have -ies as a regular plural ending. Thus fairy becomes fairies and berry becomes berries. The above are all examples of regular plurals.

Irregular plurals include words that are different in form from the singular forms and do not simply add an ending. These include me​n from man , women from woman and mice from mouse.

Some irregular plurals are formed by changing the vowel of the singular forms, as in feet from foot, geese from goose and teeth from tooth.

Some irregular plurals forms are formed by adding -en, as oxen from ox and children from child.

Some nouns ending in -f form plurals in -ves, as in loaf to loaves, half to halves, wife to wives and wolf to wolves. However, some have alternative endings; hoof changes to either hoofs or hooves, and some form regular plurals; for example roof becomes roofs.

Some irregular plurals forms are the original plural forms of foreign words adopted into English. Examples are stimuli from stimulus, phenomena from phenomenon, criteria from criterion, and larvae from larva. In modern usage there is a growing tendency to anglicise the plural forms of foreign words. Many of these coexist with the plural form, for example thesauruses and thesauri, formulas and formulae, gateaus and gateaux and indexes and indices. Sometimes, the anglicised plural formed according to the regular English rules differs slightly in meaning from the irregular foreign plural. Thus, indexes usually applies to guides in books whiles indices is usually used in mathematics.

Some nouns have irregular plurals in which the plural and the singular forms are the same. These include sheep, grouse (the game-bird) and salmon. Also some nouns have both a regular plural and an irregular form. Brother, for instance, has the plural becomes brothers and brethren, although brethren is now mainly used in a religious context and is archaic in general English.

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