1. We play basketball in the gym.

Question

2. We play basketball at the gym.
3. We play soccer on the playground.
4. We play soccer in the playground.
5. We play soccer at the playground.
6. He is playing in the room.
7. He is playing at the room.
[Can we use all the prepositions? What about #7? Do we have to use only 'in' before 'the room'?]

Answer 1

I would not use 4 or 7. The others are OK.