Question

when we use "an" is used

Answer 1

In any case, the rule is that you use "an" if the next word begins with a vowel sound. Vowel sound is crucial here because many words that begin with vowel letters do not begin with vowel sounds (e.g. user) and vice versa (e.g. hour).

Answer 2

an we can use when some sentences we can the vowels sound when it is start

for example
that is an apple