When to use an in a sentence?

Answer 1

when preceding a common noun beginning with a vowel sound (vowels are a, e, i, o, u).

For example:

an apple, an elephant, an idea, an orphan

Answer 2

Use "an" before words that start with a vowel sound, such as "an apple" or "an open door." Use "a" before words that start with a consonant sound, such as "a car" or "a book."