An

An is the form of A (the indefinite article) used before vowel sounds.

Examples:
A: an apple | an angry bird
E: an egg | an excellent book
I: an idea | an interesting job
O: an orage | an open door
U: an uncle | an ugly guy

Words like European, union, university, used, useful are pronounced with a consonant sound at the beginning, so the form a is used with them: a used car.

In words like honest, honour, honourable, hour, hourly, on the other hand, the "h" is silent. They are pronounced with a vowel at the beginning, and so the form an is used with them.
Ex: an hour ; an honest person etc.

I hope this article helps. If you have any question, please leave a comment below.