Ответ:
1. I have lost my key I can't enter my flat now.
2. I had lost my key and had walked in the yard before my mother came and opened the door.
3. Why are you late today, Pete? Frankly speaking, I have overslept.
4. Pete was late for school because he had overslept.
5. After Bill had done his homework, he began playing on a computer.
6. "Have you done your homework?" Mother asked me.
7. The dog had eaten all the sausages before we came home.
8. Have you eaten your breakfast today?
9. Jack had been to three countries by the time he was seventeen.
10. Have you been to Baikal?
11. Nick has read an interesting book about space this week.
12. He had read five textbooks by the history exam last month.
13. I looked at the new pupil in our class and realized that I had seen him before.
14. Have you ever seen a kangaroo?
15. Have you ever broken any glasses or cups? Do you believe in superstitions"?
16. The father wanted to know who had broken his favourite cup.
17. During his summer holidays Sam decided to visit the city he had lived in before