11.The key_________ she was opening the door broke.

12. The library ___________ we often go on Sundays is not far from our school.

13. No one know the reason _____________ she is leaving so early.

14. At last they came to a river _____________ a new bridge is being built.

15. He is the teacher _____ ______ help we have made great progress.

6.定语从句有限制性和非限制性两种。

限制性定语从句是先行词不可缺少的部分，去掉它主句意思往往不明确,它对主句起到限制作用。

非限制性定语从句是先行词的附加说明，去掉了也不会影响主句的意思，它对主句起进行补充说明，它与主句之间通常用逗号分开。如果先行词是专有名词或独一无二的事物，我们一般要用非限制定语从句。　　　

This is the house which we bought last month. 这是我们上个月买的那幢房子。（限制性）

The house, which we bought last month, is very nice.这幢房子很漂亮，是我们上个月买的。（非限制性）

注意事项：

1.只能用which不能用that的情况:

① 引导非限制性定语从句

② 先行词在从句中作介词的宾语且介词前置, 即当关系词紧跟在介词后面时,如：

This is the house in which Lu Xun lived.==This is the house which/that Lu Xun lived in.

③先行词本身是that:

The clock is that which tells the time.

练习：将下列句子合并成用关系副词引导的定语从句。

1. He is the doctor. He operated on me last year.

2. Is he the man? You are waiting for the man.

3. The girl is called Mary. I saw her yesterday.

4. The girl is from China. We all think highly of her.

5. This is the film star. His name is well known all over the world.

6. That is the room. Its door needs painting.

7. The house is Tom's. The door of the house faces south.

8. The man is a doctor. He lives opposite our house.

9. Do you know the man? He is talking to your teacher.

10．This is the most exciting moment. I will never forget it.

11．Is the book difficult? You read the book this morning.

12． The letter is from my uncle. I received it yesterday.

13．The teacher saw what happened. He was standing nearby.