Possible Duplicate:
When should I use “a” vs “an”?
Does one use 'a' or 'an' before the word X-Ray?
As far as I understood, you prefix "an" instead of "a" before any proceeding word starting with a vowel when you wish to speak about an object (there's probably a correct term for this), but ie. a towel, a tree etc.
So why is it so common to say "a user clicked this" or "please enter a username" and not "an" instead of "a" like "an umbrella"?
