Indefinite Articles: A and An

An

Use an when the indefinite article comes before a word beginning with a vowel sound

Examples:

An operation

An idea

An apple

Subtopic

Subtopic

A

Use a when the indefinite article comes before a word beginning with a consonant sound

Examples:

A toy

A book

A house

The two indefinite articles in English are a and an. The indefinite article an is used to make pronunciation easier when reading a text aloud. The general rule is to use a when the indefinite article precedes a word beginning with a consonant sound and an when it precedes a word starting with a vowel sound.