參考答案：

　　1. The girl was knocked down by a bus when she was crossing the road.

　　2. Following these instructions, you will soon learn how to use this computer.

　　3. In western countries, children have to live on their own as soon as they reach the age of 18.

　　4. Compared with neighboring small stores, supermarkets are more convenient for residents to shop.

　　5. I will remember the happy time we spent together forever.

　　6. Those who pretend to know what they don't know will pay a price for it one day.

　　7. This cabinet is too expensive. Besides, it occupies too much room.

　　8. The mother thanked Tom for having saving her drowning son.

　　9. Teachers should know how to help students develop mentally and physically.

　　10. I find it necessary for one to have a hobby.

　　11. Modern industry is to blame for all sorts of pollution it has caused.

　　12. According to the law, Internet cafes are inaccessible to anyone under the age of 18.

　　13. All kinds of reference books are available to us in our school library.

　　14. The child is so naughty that he upsets his parents who are busy with their work.

　　15. She used to live just around the corner, didn’t she?

　　16. I take it seriously, for it's a matter of principle.

　　17. The road gradually widened until we found ourselves in a large valley.

　　18. Mother went shopping; meanwhile, I cleaned the house.

　　19. Conservation is one way to make sure we have enough energy in the future.

　　20. It was because the economic crisis lasted for several years that the government’s economic policy dimmed the popularity of the president.