A plural noun names more than one thing. We add -s or -es to a noun to make a regular plural noun.
Here are some regular plural nouns:
dog – dogs
cat – cats
patch – patches
class – classes
An irregular plural noun does not become plural by adding -s or -es.
An irregular plural noun completely changes its spelling when the noun names more than one thing.
Some irregular plural nouns change -oo to -ee:
one foot – two feet
one goose – two geese
one tooth – two teeth
Nouns ending in -f or -fe change to a -v when plural:
half – halves
calf – calves
wife – wives
knife – knives
Nouns ending in -us change to ending in -i when they are plural:
one cactus – many cacti
one fungus – several fungi
Certain nouns become plural with no real pattern for how they change their spelling:
one mouse – three mice
one man – two men
one woman – two women
Irregular plural nouns change their spelling when the noun names more than one thing.