在复合句中，修饰某一名词或者代词的从句叫做定语从句。如：The story (that) you read is The Resue.定语从句放在先行词的后面。引导定语从句的词有关系代词 that , which , who (宾格whom，所有格whose)和关系副词 where, when, why。关系代词或关系副词在定语从句中充当一个成分。

一、关系代词（人，物）

1、This is the man (who) helped me . 主语，人

2、The girl (whom) I met looks like Lily. 宾语，人

3、A plane is a machine (that) can fly. 主语，宾格，人，物

4、This is a book (which) you want. 主语，宾格，物

5、The room (whose) window is red is mine. 主语，宾语，人，物

二、关系副词（状语）

1、It happened on the day （when） I was out. 时间

2、I recently went to the town (where) I was born. 地点

3、The reason (why) he didn't come was that he missed his train. 原因

初中的，应该就只学关系词引导的定语从句吧，好好理解上面，应该也就可以了。不难的，努力哦@@～