Everytime you have a vowel (a, e, i, o, u, y) at the beginning of a word, you use 'an'.
For example:
An apple
An elephant
An Iguana
An omelette
An uzo
An year
There are few exeptions too:
An hour
Everytime you have a vocal (a, e, i, o, u, y) at the beginning of a word, you use 'an'.
For example:
An apple
An elephant
An Iguana
An omelette
An uzo
An year
There are few exeptions too:
An hour