'A' and 'an' are articles used with singular nouns.

The article 'a' is used with singular nouns that do not begin with a vowel. 

a motorbike

a cat

a cup of tea

a call

'an' is used with singular articles that do begin with a vowel (a, e, i, o, u)

an aeroplane

an umbrella

an orange juice

an elephant

Watch our video for an explanation of when to use the prepositions 'a' and 'an' in English.