1. We couldn't answer those two difficult questions.

• Correct: Those two questions were too difficult for us to answer.

2. They couldn't drive because of the fog.

• Correct: The fog prevented them from driving.

3. I haven't read a book on English grammar for a long time.

• Correct: It's been a long time since I read a book on English grammar.

4. He drives very carefully.

• Correct: He is a careful driver.

5. My father usually has me clean his bicycle.

• Correct: My father usually makes me clean his bicycle.

6. The postman was bitten by our dog.

• Correct: Our dog bit the postman.

7. That factory is producing more and more pollution.

• Correct: More and more pollution is being produced by that factory.

8. He won't buy you a palace because he is not a millionaire.

• Correct: If he were a millionaire, he would buy you a palace.

9. A guard was protecting the jewels.

• Correct: The jewels were being protected by a guard.

10. I don't find it difficult to study late at night.

• Correct: I am used to studying late at night.

11. You can't drive unless you have got a license.

• Correct: If you don't have a license, you can't drive.

12. I will meet her at the airport.

• Correct: She will be met by me at the airport.

13. She hasn't visited her grandfather for two weeks.

• Correct: It is two weeks since she last visited her grandfather.

14. It's such a heavy box that I can't lift it.

• Correct: The box is too heavy for me to lift.

15. He spent two hours a week sorting out stamps.

• Correct: Sorting out stamps took him two hours a week.

16. The garage is servicing my car on Monday.

• Correct: I am having my car serviced at the garage on Monday.

17. I regret spending so much money.

• Correct: I wish I hadn't spent so much money.

18. How long is it since they moved here?

• Correct: When did they move here?

19. He didn't pass the exam because he didn't work hard.

• Correct: If he had worked hard, he would have passed the exam.

20. Sally's parents gave her a bicycle for her birthday.

• Correct: Sally was given a bicycle for her birthday by her parents.