1. Singulars nouns that end in -f or -fe change to -ves in their plural form: elf/elves wolf/wolves life/lives shelf/shelves wife/wives
2. Some singular nouns have their own plural form:
mouse/ mice (wo)man/ (wo)men foot/feet tooth/teeth
3. Some singular nouns don't change when you make them plural:
fish/fish sheep/sheep deer/deer jeans/jeans