The spelling conventions for singular and plural nouns

The spelling conventions for singular and plural nouns
The rule: Regular nouns form their plural by adding “s” or “es” to the singular. Irregular nouns form their plural by changing their endings in a variety of different ways or remaining the same in both singular and plural.

To make regular nouns plural, add ‑s to the end.
cat – cats
house – houses
dog – dogs
fish – fishes

Irregular nouns form their plural in the following ways:
If the noun ends with ‑f or ‑fe, the f is often changed to ‑ve before adding the -s to form the plural version.
elf – elves
loaf – loaves
wife – wives
wolf – wolves

Exceptions:
roof – roofs
belief – beliefs
chef – chefs
chief – chiefs
If a singular noun ends in ‑y and the letter before the -y is a consonant, change the ending to ‑ies to make the noun plural.
city – cities
puppy – puppies

If the singular noun ends in -y and the letter before the -y is a vowel, simply add an -s to make it plural.
ray – rays
boy – boys

If the singular noun ends in ‑o, add ‑es to make it plural.
potato – potatoes
tomato – tomatoes

Exceptions:
photo – photos
piano – pianos
halo – halos

With the unique word volcano, you can apply the standard pluralization for words that end in -o or not. It’s your choice! Both of the following are correct:
volcanoes
volcanos

If the singular noun ends in ‑us, the plural ending is frequently ‑i.
cactus – cacti
focus – foci

If the singular noun ends in ‑is, the plural ending is ‑es.
analysis – analyses
ellipsis – ellipses

If the singular noun ends in ‑on, the plural ending is ‑a.
phenomenon – phenomena
criterion – criteria

Some nouns don’t change at all when they’re pluralized.
sheep – sheep
series – series
species – species
deer –deer

You need to see these nouns in context to identify them as singular or plural. Consider the following sentence:

Mark caught one fish, but I caught three fish.

Plural Noun Rules for Irregular Nouns
Irregular nouns follow no specific rules, so it’s best to memorize these or look up the proper pluralization in the dictionary.
child – children
goose – geese
man – men
woman – women
tooth – teeth
foot – feet
mouse – mice
person – people