Noun Part I (Count Noun Singular and Plural)

Count nouns have two forms: singular and plural.

The singular form refers to one person or thing:

• a book; a teacher; a wish; an idea

The plural form refers to more than one person or thing:

•  books; teachers; wishes; ideas

Singular count nouns

Singular count nouns cannot be used alone. They must have
a determiner:

• the book; that English teacher; a wish; my latest idea

or a quantifier:

• some new books; a few teachers; lots of good ideas

or a numeral:

• two new books; three wishes

Plural forms:

We usually add –s to make a plural noun:

book > books; school > schools; friend > friends

We add -es to nouns ending in –ss; -ch; -s; -sh; -x

class > classes; watch > watches; gas > gases; wish > wishes; box > boxes

When a noun ends in a consonant and -y we make the plural in -ies...

lady > ladies; country > countries; party > parties

…but if a noun ends in a vowel and -y we simply add -s:

boy > boys; day > days; play > plays

Some common nouns have irregular plurals:

Man > men; woman > women; child > children; foot > feet;
person > people

Plural count nouns do not have a determiner when they refer to people or things as a group:

Computers are very expensive.
Do you sell old books?

 Source : British Council