2）用which而不用 that的情况：①引导非限制性定语从句；②代表整个主句的意思；③介词 + 关系代词。
e.g. He had failed in the maths exam, which made his father very angry. 他数学考试没有及格，这使他的父亲很生气。
This is the room in which my father lived last year. 这是父亲去年居住过的房子。
3）as引导定语从句时的用法
①as引导限制性定语从句通常用于the same … as, such … as结构中。
e.g. I want the same shirt as my friend’s. 我要一件跟我朋友一样的衬衫。
Such machines as are used in our workshop are made in China. 我们车间使用的这种机器是中国制造的。
②as引导非限制性定语从句既可放在主句之前，也可放在主句之后，用来修饰整个句子。通常用下列句型：as is known to all, as is said, as is reported, as is announced, as we all know, as I expect 等。
e.g. As I expected, he got the first place again in this mid-term examination. 正如我所预料的那样，他在这次期中考试中又获得了第一名。
3）as 引导非限制性定语从句时与which的区别
①当主句和从句语义一致时，用as；反之，用which来引导非限制性定语从句。
e.g. He made a long speech, as we expected.
He made a long speech, which was unexpected.
②当非限制定语从句为否定时，常用which引导。
e.g. Tom drinks a lot every day, which his wife doesn’t like at all.
2. 关系代词在从句中作主语时，从句的谓语动词单数还是用复数应由先行词决定。
e.g. The man who lives downstairs speaks English fluently. 住在楼下的那个人英语说得很流利。
The students who are in Grade Three are going to climb the hill tomorrow.