Introduction:

‘A/an’ are indefinite articles. ‘The’ is a definite article.

Structure:

Use ‘a’ before a noun beginning with a consonant and ‘an’ before a vowel. Remember a long u (pronounced ‘you’) such as university uses “a” (university, union, universe, unique etc…).

Examples:

I am moving to a new city.
She is going to the head office.
They have an important meeting.
They have a unique strategy.