'a' if indefinite or "the" if definite.
Examples
My daughter wants
to buy a dog this weekend. (Indefinite-It could be any dog)
The dog in the backyard is very cute.(Definite- The one in the backyard) 1. He requested a puppy for his birthday.
2. He wanted the puppy he played with at the pet shop.
3. She ordered a hamburger without onions.
4. Did you drink the coke I just ordered?
Rules of Articles