定语从句由关系代词who、whom、whose、which、what、as和关系副词where、when、why等引导，但须记住：
1.what不能引导定语从句.
2.关系词的分析须考虑它在定语从句中的成分。
一.指人的关系代词有who、whose、whom、that. 试分析：
The old man who/whom /that we visited yesterday is a famous artist.
Miss Wang is taking care of the child whose parents have gone to Beijing .
The man with whom my father shook hands just now is our headmaster. （=The man who/whom that my father shook hands with just now is our headmaster.）注：
A． 指人时有时只用who不宜用that。
1．先行词为one、ones或anyone
（1）The comrade I want to learn from is the one who studies hard and works hard.
（2）Anyone who breaks the law should be punished.
2．先行词为these时
These who are going to Beijing are the best students of our school.
3．在there be 开头的句子中
There is a student who wants to see you.
4．一个句子中带有两个定语从句，其中一个定语从句的关系代词是that，另一个宜用who，以免重复。
The student that won the first prize is the monitor who works hards.
5．在非限制性定语从句中
A。I met a friend of mine in the street, who had just come from Japan.
B．主句以who开头的句子中,只用that,不用who。
二．指物的关系代词有which、whose(=of which)、that. 试分析：
1. I like the books which / that were written by Lu Xun.
2. The desks (which/that) we made last year were very good.
3. This is the house in which we lived last year. (= This is the house which /that we lived in last year.)
4. I live in the room whose windows face south. (= I live in the room ,the windows of which face south.)
注：
A. 介词如果位于作为其宾语的关系代词之前时，只能用whom、 which不能用who或that.
(1). The girl about whom they were talking is our monitor.
(2). The book in which there are many interesting things was written by Li Ming.
B. 部分短语动词中的介词不可与动词拆开，在定语从句中其介词不可前置，只能放在其动词之后。
Is this the book which she is looking for?
The old man who/whom they are waiting for is Professor Li.
The child who/whom she is looking after is Wang Ping’s son.
C． 指物时，下列情况下只能用that ，不宜用which。
追答as与which引导的定语从句
两者均可引导非限制性定语从句，有时可以互换，但下列情况多用as。
1. 关系代词引导的定语从句居句首时。 As we all know, the earth is round.
2. 当与such或the same连用时，一般用as。 Such books as you tell me are interesting. I have the same plan as you.
3. 当从句和主句语义一致时，用as，反之则用which。 She has married again, as was expected. She has married again, which was unexpected.
4. as在从句中作主语时，后面常接行为动词的被动语态，如be known，be said，be reported等，如从句中行为动词是主动语态，一般要用which作主语。
She has been late again, as was expected.
Tom has made great progress, which made us happy.