1. Polly has just brought Coca-Cola.
2. Ron hasn't bought his Halloween costume yet.
3. Have they bought flowers yet?
4. Have they made sandwiches yet?
5. They have just bought hot dog.
6. Andrew have already brought a DVD player.
7. They have already decorated the room.
Во втором предложении вопросительного знака не стояло? Если мы употребляем слово yet, то предложение либо является вопросом, либо это отрицательное предложение.