The plural of a nounA part of speech that names a person, place or thing. E.g.: The koala is a marsupial. ("koala", "marsupial") is usually formed by adding -s to the singular form:

mistake, mistakes

For certain nouns it is true that if the singular noun ends in -f or -fe, the plural ending is -ves:

elf, elves
knife, knives
thief, thieves
hoof, hooves

However, there are nouns which do not follow this rule:

cliff, cliffs
chief, chiefs
cuff, cuffs
roof, roofs

A number of nouns have irregular plural forms:

child, children
ox, oxen

foot, feet
goose, geese
tooth, teeth

mouse, mice
louse, lice

man, men
woman, women

Nouns of Latin or Greek origin may keep their original plural forms:

crisis, crises
bacterium, bacteria
appendix, appendices
formula, formulae

Nouns with the same singular and plural forms
Countable and uncountable nouns The subjunctive