Articles (A and The) with buildings
These are public buildings: school, college, university, prison, hospital and church. We don't say "a" or "the" when we are talking about their normal purpose (for students, criminals, sick people, religious people).
She was in hospital for 3 weeks with a broken leg.
But, if we are just talking about the building, or if we use an adjective, we do say "a" or "the", using the normal rules.
I parked my car outside the hospital.
She was in a really good hospital for 3 weeks.
Places in a town
For most shops or places in a town, we say "go to the" (because we imagine that there is only one, or that the listener knows which one we are talking about).
I'm going to the pet shop.
But, again, we can say "a" or nothing when we are just talking about the building or using the normal rules.
There's a pet shop near here.
I love pet shops.
From: The Tiny TEFL Teacher