用來說明主句中某一名詞或代詞（有時也可說明整個主句或主句中一部分）而起定語作用的句子叫作定語從句。
一、關係代詞引導的定語從句
1. 關係代詞用來指代先行詞是人或物的.名詞或代詞
句子成分 用於限制從句或非限制性從句 只用於限制性從句
代替人 代替物 代替人或物
主語 Who which that
主語 Whom which that
賓語 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.關係代詞的用法
（1）如果先行詞是all，much，anything，something，nothing，everything，little，none等不定代詞，關係代詞一般只用that，不用which.例如：
All the people that are present burst into tears.
