'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 cup of tea
'an' is used with singular articles that do begin with a vowel (a, e, i, o, u)
an orange juice
Watch our video for an explanation of when to use the prepositions 'a' and 'an' in English.