Whenever I am writing something about users/units, I always want to say 'a user'/ 'a unit'...but if 'an' is supposed to be used before vowels, how come 'an user' and 'an unit' don't quite sound right?

..in fact, it's down right hard to say!


...Is there some sort of rule with the 'an' and 'a'?


...I know this is REALLY off topic, but this question has been bugging me for weeks...someone help?
