1．定语从句在句中作定语用，修饰句中的某一名词（或代词），被定语从句所修饰的词叫先行词。定语从句必须放在先行词之后。引导定语从句关联词有：
关系代词：who，whom，whose，which，that
关系副词：when，where，why, that等。
⒈定语从句试题，主要考查使用关系词的能力。
○1找出被修饰的先行词，分析关系词在从句中的语法作用，确定关系代词：
This is the hospital（that）we visited yesterday．
此句的先行词为hospital,可用which／that，但不可用who，whom。关系词在从句中作visit的宾语，不可用where。
又如：We visited a factory＿which or that＿makes toys for children.
We visited the factory __where__ Li Ming’s father once worked.
○2掌握 which和that使用上的区别。
a. 只能用which的情况：
（l）先行词是物时，介词后的关系代词应用which，不用that。
This is the home in which（＝where）Lu Xun once lived.
若介词不前置,用that也可。
如：The room which／that we live in is clean．
（2）在非限制性定语从句中，关系代词用which．
Last night I saw a very good film, which was about the Anti-Japanese War.
（3）先行词为物，且被that或those修饰时，通常用which。如：
That book which Tom bought yesterday is useful.
b.在下列情况下，一般用that。
（1）先行词为all, everything, nothing，something，anything，little，no，much，the only，the same，the very，the first，the last，any，every时。如：
Is there anything（that）1 can do for you in town？
He was the only teacher（that）I knew in your schoo1．
（2）先行词为形容词最高级所修饰时。
This is one of the most exciting football games (that) I have ever seen.
It was the finest art exhibition that we ever saw.
先行词中既有人，也有物时。如：
Can you see a man and his horse that are crossing the bridge?
○3用词不可重复。定语从句中，己有关系词作主语、宾语或状语时，不可再重复。如：
The man＿＿＿＿was a friend of mine！
A. that you just talked to B. whom you just talked to him
C. who you just talked to him D. which you just talked to
○4as 和which可代表全句意思，但其用法有差别。它们引导非限制性定语从句，可以用整个主句作为先行词，指代上面或下面所说的一件事。区别如下：
a. as引导的从句可放在主句之前或之后，而which只可放在主句之后。如：
As is known to us all, we have made great progress in science and technology.
Tom failed in the exam, which made us so sorry.
b. 从句中的谓语动词是describe, do, tell, know, expect，see, hear, watch, say, show等实义动词时，用as.
As we all know, Shakespeare is a famous writer.
To shut your eyes to facts, as many of you do, is foolish.
There is great disorder, as the newspapers will have told you.
c. 当从句的谓语动词是否定形式或从句谓语带有一个复合宾语结构时，一般用which而不用as。
如：
You pretended not to know me, which I didn’t understand.
He admires everyone in the class, which I find quite strange.
d. as 作关系代词常与the same, such配合使用。
如：
I had the same things as you.
Metals are such things as iron, silver and gold.
○5 介词加关系代词中介词的选用
（1） 根据与先行词的习惯搭配。
I will never forget the day on which I joined the Party.
（2） 根据和从句中动词的固定搭配。
This is not the dictionary of which the teacher spoke in class.
（3） 根据句子的具体意义。
He invented a telescope, with which he could study the sky.
○6使用定语从句应注意的几个问题：
（1）关系代词在从句中作主语时，从句的谓语动词用单数还是复数应由先行词决定。
The mm who lives downstairs speaks English fluently.
The students who are in Grade Three are going to climb the hi11 tomorrow。
(2)定语从句有时不直接紧靠着先行词，中间由一个定语、状语或谓语隔开。
如：
There is an expression in his eyes that I can’t understand．
The days are gone forever when the Chinese people were looked down upon。
(3)引导定语从句的关系副词可以用“介词+which”来代替。
如：
0ctober l，1949 was the day on which（=when）the People’s Republic of China was founded.
(4)当定语从句中谓语动词是带介词或副词的短语动词时，短语动词的各个固定的部分不要拆开。
The dictionary which you are looking for is on the desk.
The sick man whom she is 1ooking after is her father.