The Indefinite Article

While the definite article in English remains constant at all times, the indefinite article a changes to an before any word beginning with a vowel (a, e, i, o, u) or a vowel sound. Examples:

a book

an egg

a girl

an old friend

an hour

an honest

Note that before such words as hour, honest, honor, we an instead of a because the initial h is silent and the word thus begins with a vowel sound. Likewise, before words like university, union, one we use a instead of an because, although these words begin with vowels, the first sound is a consonant sound, namely (y), (w), etc.