Q:

question about when we use a and when we use an

The patient had been waiting in the emergency room for almost an hour before a doctor finally treated her.
(an) goes with a,e,i,o,u
(a) goes with the other letters
why do we use an hour ? instead  of a ?

A:

Because the h in the word hour (click on the link if you want to hear how native speakers pronounce it) is silent. In other words, it's not pronounced. What you're effectively saying is our. And since our begins with a vowel, you need to precede it with an an.

