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

Question

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:

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

I tried to add a consonant tag, but their isn't one and I don't have enough rep! — BG100, May 06 '11 at 09:56
Possible duplicates: A user or AN user,Use of A versus AN, When to correctly use AN and A Read the first question I linked, it's the exact duplicate of yours. :) — Alenanno, May 06 '11 at 10:00
@Alenanno: I searched the site for keywords A and AN, but it didn't return anything!! Not sure why, but thanks for the link. — BG100, May 06 '11 at 10:03
the site search doesn't really work for such short and general words. Entirely not your fault. What I can recommend instead is having a look at the "faq" tab under "Questions" (where 3 out of the top 5 questions deal with a vs an), or googling using the "site:" operator. — RegDwigнt, May 06 '11 at 10:37

score 3 · Accepted Answer · answered May 06 '11 at 10:00

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":

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.

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