Irregular plural forms

The plural of a noun is usually formed by adding -s to the singular form:

mistake » mistakes

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

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

But 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