how to use a and an in english?

use a : we use a when in the first word is consonant word
use an : we use an when in the first word is vocal word

choose the sentence with an and a !!!

1. it is (a\an) pencil
2. he is (a\an) policeman
3.i have (a\an) axe
4. it is (a\an) airplane
5.he is (a\an) teacher