"A" becomes "an" before a word that starts with a:

1. English has two indefinite forms, "a" and "an". 2. "A" is rendered as "an" when followed by a vowel sound. 3. It depends on sound, not on word length or capitals. **Why not the others:** Word length, capital letters and silent letters do not decide between a and an; the vowel sound does. _Source: Wikipedia, "Article (grammar)", section "introduction" (revision 1378409904)_