There are two numbers in English – singular and plural.

A noun that represents one person or thing is said to be in the singular number. Examples are: apple, mango, cat, cow, cock, book, pen, table, chair etc.

A singular noun can be used with the indefinite article a/an.

A noun that represents more than one person or thing is said to be in the plural number. Examples are: apples, mangoes, cats, cows, cocks, books, pens, tables, chairs etc.

A plural noun cannot be used with the indefinite article a/an.

Related posts:

1. Noun: Gender
2. Proper Nouns and Common Nouns
3. Collective Nouns and Abstract Nouns