Nouns can be singular or plural. When you are talking about one person, animal, place, or thing, use a singular noun.
an airplane a letter
a bicycle a map
a boy a photograph
When you are talking about two or more people,animals, places, or things, use plural nouns.
Most nouns are made plural by adding -s at the end.
Some plural nouns end in -es.
Some plural nouns end in -ies.
What if there is a vowel before the y? In that case, add -s to form the plural.
If a noun ends in -f, you often change f to v, and add -es.
With some words that end in -fe, you change f to v, and add -s.
If a noun ends in -o, you just add -s to form the plural.
a hippo hippos
a video videos
But with some nouns that end in -o, you add -es to form the plural.
a tomato tomatoes
a potato potatoes
Some plural nouns don’t follow the -s rule. They don’t end in -s, -es, -ies or -ves. Instead, the word changes form.
Some plural nouns are the same as the singular noun.
Some nouns are always plural.
Kaynak: Basic English Grammer Book1