用来说明主句中某一名词或代词(有时也可说明整个主句或主句中一部分)而起定语作用的句子叫作定语从句。
一.词引导的定语从句
1. 关系代词用来指代先行词是人或物的名词或代词
句子成分 用于限制从句或非限制性从句 只用于限制性从句
代替人 代替物 代替人或物
主语 Who which that
主语 Whom which that
宾语 Whose(=of whom) whose(=of which)
例1：This is the detective who came from London.
例2：The book which I am reading is written by Tomas Hardy.
例3：The desk whose leg is broken is very old.
例4：This is the room that Shakespeare was born in.
2.关系代词的用法
(1) 如果先行词是all, much, anything, something, nothing, everything, little, none等不定代词，关系代词一般只用that，不用which。例如：
All the people that are present burst into tears.
(2) 如果先等词被形容词最高级以及first, last, any, only, few, mush, no, some, very等词修饰，关系代词常用that,不用which, who，或whom。例如：
(3) 非限制性定语从句中，不能用关系代词that,作宾语用的关系代词也不能省略。例如：
There are about seven million people taking part in the election, most of whom、are well educated.
(4) which还有一种特殊用法，它可以引导从句修饰前面的整个主句，代替主句所表示的整体概念或部分概念。在这种从句中，which可以作主语，也可以作宾语或表语，多数情况下意思是与and this 相似，并可以指人。例如：
He succeeded in the competition, which made his parents very happy.