What is the rule for using “a” or “an” in a sentence? [duplicate]

呢篇講的問題比較簡單,但答案指出了y-glide這個consonant之特別:

問: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.

答:

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.