Ответ:
1. I have just cleaned the floor.
2. She hasn't been eating anything for six hours.
3. She has been studying for four hours.
4. I have seen it twice already.
5. Sandy has been teaching English for 10 years.
6. Chris hasn't done the washing up yet.
7. He has been walking all morning.
8. They have been playing in the garden for two hours.