What is the rule for using "a" or "an" in a sentence? [duplicate]
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.