Irregular plural (1)

There are many types of irregular plural, but these are the most common:

When a noun:

Ends with - fe
Change f to v then Add - s

knife - knives / life - lives / wife - wives

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Ends with - f
Change f to v then Add - es
half - halves / wolf - wolves / loaf - loaves

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Ends with -o
Add - es
potato - potatoes / tomato - tomatoes / volcano - volcanoes

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ends with -us
Change -us to -i
cactus - cacti / nucleus - nuclei / focus - foci

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ends with -is
Change -is to -es
analysis - analyses /crisis - crises / thesis - theses

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ends with - on
Change -on to -a
phenomenon - phenomena / criterion - criteria

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ALL KINDS
Change the vowel or Change the word or Add a different ending

man - men / woman, women / foot - feet

child - children /person - people

tooth - teeth /mouse - mice / louse, lice

Unchanging
Singular and plural are the same.

sheep - deer - fish (sometimes)

The following nouns have no singular:
scissors
oats
tongs
dregs
trousers
pinchers
bellows
snuffers
cattle
shears
measles
mumps
victuals
tweezers
vespers


Some nouns are always singular. Some of these nouns may be used in the plural when different kinds are meant as sugars, coffees, cottons
gold
silver
wheat
corn
molasses
copper
sugar
cotton