How to use Singular and Plural Nouns
Nouns can be singular or plural. When you are talking about one person, animal, place, or thing, use a singular noun.
a pencil case
an air balloon
Use a or an before singular nouns.
Use an before words beginning with vowels (a, i, u, e, o).
But some words don’t follow this rule. For example, use a (not an) before these words that begin with u:
Use a before words beginning with the other letters of the alphabet, called consonants.
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. When the last letters of singular
nouns are ch, sh, s, ss or x, you usually add -es to form the plural.
Example : watches, branches, dresses, foxes
Nouns like these are made plural by changing y to i, and adding -es. See example below
Example ; butterflies, flies, stories, strawberries
What if there is a vowel before the y? In that case, add -s to form the plural.
Example : monkeys, turkeys, toys, donkeys
Source : Basic English Grammar, for English Language Learner by Anne Seaton & Y.H. Mew