在复合句中，修饰某一名词或代词的从句叫做定语从句。
定语从句所修饰的词 叫先行词。 定语从句放在先行词后面。定语从句分为限定性定语从句和非限定性定语从句。本章只介绍限定性定语从句。引导定语从句的词有关系代词that、which、who、 which、 whose和关系副词where 、when，关系代词和关系副词是引导词， 在定语从句中担当某一成份。
关系代词和关系副词的作用：
先行词指物时， 关系代词用that 或which（在句中作主语或宾语）。
如：China is a country which / that has a large population.
The computer game which/ that he is playing is his favorite.
先行词指人时， 关系代词用（who）作主语或宾语，whom (作宾语)，whose (作定语)，that (作主语或宾语)。 如：
The man who is mending the machine has been retired.
This is the policeman who/ whom they were talking about yesterday.
I like the girl whose mother is an actor.
注意：下列情况下关系代词只用that 而不用which 或 who。
先行词是指物的不代词，如：all, little, few, much, none, anything, nothing, everything等时。如：I didn't understand the words all that he said.
2．先行词由最高级、序数词以及any, very, only, all, no, just 等所修饰。如：This is the most beautiful place that I have been to .
行词既包括人，也包括物。如：They always like to talk about the man and the things that they see on the road.
当主句是以which 或who 开头的特殊疑问句时。如：who is the man that opened up the lab.
例题解析：举一反三，学的更轻松！
He is a friend ______ can help you in time of need.
A. who he B. whose C. / D. who
解析：引导词在定语从句中做主语，指人。 应选D 。
The most important thing ______ we should pay attention to is the first thing ______ I have said.
A. which, that B. that, which C. which, which D. that, that
解析：先行词前有序数词， 最高级时，引导次只能用 that. 应选D.
This is the only one of these books that ______worth reading.
A．has B. have C. is D. are
解析：引导词that在句中作主语，代指one ，应选C。
The man ______ talked to you just now is my father.
A. who B. he C. which D. whose
解析：引导词在句中作主语指人，应选A.
They thought too much about ______.
A. which I had said B. what I had said
C. that I had said D. I had said
解析：此题引导词作在从句中作宾语，表示说话的内容，应 选B。
标签：从句
