Chuyển sang câu bị động
11. They rub pieces of bamboo together to make a fire
12. Will you invite your friends to your birthday party?
13. We can't finish all my homework soon
14. People say that he works very hard
15. Do they speak English at scholl?
16. Has some one posted the letters?
17. Some one has already told him the whole truth
18. People are spending a lot of money on food now
19. Why don't they mend the roof before is falls in
20. Visitors must leave umbrellas and sticks in the cloakroom