1) Polly has just brought Coca-Cola.
2) Ron hasn't bought his Halloween costume yet. или это вопрос, в утверждении yet не употребляем
3) Have they bought flowers yet?
4) They have just bought hot dogs.
5) They haven't made sandwiches yet. или это вопрос, в утверждении yet не употребляем
6) Andrew has already brought a DVD player.
7)They have already decorated the room.