Indefinite articles a/an

We use a + consonant sound – a book, a hospital, a [ju:] university, a [ju:] uniform

We use an + vowel sound – an apple, an hour, an heir, an interesting book, an [A] ugly toy

1. We use a/an with a singular noun when we mention something for the first:
I saw a dog in the street. I bought a pen and a pencil yesterday.

2. One of many:
There is a shop in my district. There’s a new cinema in Al-Farabi street.

3. Any or it doesn’t matter which one:
Give me a pen. Can you lend me a copybook? I’m eating a peach.

4. Every/each/per:
Five pounds (per) a kilo. I go to the barber’s (every) once a month. I drink two liters of water (every/each) a day. I never drive at more than 100 kilometers (per) an hour.

5. Description (who is who, what is what):
He is a doctor. She is an optimist. I’m a journalist. It’s a table. This is a famous building.

6. To describe something or someone with to be or to exclaim:
That’s a good book. It’s a nice day today.

Such a great day! What a stupid mistake!

But

Such a clever children! What an awful whether!