定语从句在句中作定语，修饰句中的名词或代词，被修饰的词称为先行词。定语从句必须紧接被修饰的先行词，引导定语从句的关联词有关系代词（ who , whom , which 和 that ）和关系副词（ when , where , why )。

初中阶段只要求掌握由关系代词 who , which 和 that 引导的定语从句。

关系代词在定语从句中可以作主语或宾语等，同时，它们又代表定语从句所修饰的名词或代词，即先行词，例如：

做主语

who . that 代表人

which，that 代表物

做宾语

who ( m ). that 代表人

which，that 代表物

注:在口语中经常用who代替whom。

The girl who was injured in the accident is now in hospital .

Here ' s a photo which / that shows my car .

The people ( wha / whom / that ) we met at the party were very friendly .

Have you found the keys ( that / which ) you lost ？

注意事项：

1，先行词由形容词最高级作定语时，后面的关系代词必须用 that ，而不能用 which ，例如：

This is the most interesting book that has appeared this year .( 正)

This is the best film which he has ever seen .( 误）

2．在 something , anything , everything , nothing , only , little , all , any , much , first , last 等词后的关系代词常用 that ，而不用 which ，例如：

He was the first man that swam across the river .( 正)

Is there anything which I can do to help ?（误）

3．先行词既有人又有物或事件时，关系代词用 that ，例如：

They are the people and things that amuse him most .

4．关系代词在定语从句中起宾语作用时，在口语中往往省略，做主语时不能省略，例如；

This is the teacher who taught us English last year .( who 在从句中作主语，不能省略）

This is the place ( that ) we visited last year .

( that 在从句中作宾语，可以省略）