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.
Example :
a glass
a cup
a book
an orange
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).
Example :
an apple
an eagle
an umbrella
an igloo
an apricot
an eraser
But some words don’t follow this rule. For example, use a (not an) before these words that begin with u:
Example :
a uniform
a university
Use a before words beginning with the other letters of the alphabet, called consonants.
Example :
a basket
a spoon
a hill
a rainbow
a watch
a zoo
Example :
an heir
an honor
an hour
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