word choice - What is the rule for using "a" or "an" in a sentence?

Possible Duplicates:
“A user” or “an user”?
Use of “a” versus “an”

If I remember correctly back to my school days, the rule is to use "a" if the next word starts with a consonant, or "an" if the next word is a vowel.

For example:

• This is a banana.


• This is an egg.

If the above is correct, then why does this sentence sound wrong...?

• The account requires an username.

Answer

When a word begins with a u, sometimes it a acquires what linguists call a "y-glide": a pronunciation that makes it sound like it begins with a "y":

• user (yoozer)


• uniform (yooniform)


• ubiquitous (yoobiquitous)

And so on.

Now think of words you pronounce that begin with "y": a youth, a yew — you wouldn't say "an youth" or "an yew".

So we say "a user" but "an understanding" — just that simple.