Explanation
How to Use A and An:
v A is used with consonant sounds.
v An is used with vowel sounds.
v a + singular noun beginning with a consonant: a boy; a car; a bike; a zoo; a dog
v an + singular noun beginning with a vowel: an elephant; an egg; an apple; an idiot; an orphan
v a + singular noun beginning with a consonant sound: a user (sounds like 'yoo-zer,' i.e. begins with a consonant 'y' sound, so 'a' is used); a university; a unicycle
v an + nouns starting with silent "h": an hour
v a + nouns starting with a pronounced "h": a horse