定语从句也称关系从句、形容词性从句，一个句子跟在一名词或代词（先行词）后进行修饰限定，就叫做定语从句。

通俗来讲，从句在整个句子中做定语，这个从句就叫做定语从句。从句在主句中充当定语成分，被修饰的词叫先行词。定语从句不同于单词作定语的情况，它通常只能放在被修饰的词（即先行词）之后。定语从句由关系词（关系代词、关系副词）引导，关系代词、关系副词位于定语从句句首。

1、关系代词引导的定语从句①who, whom, that这些词代替的先行词是人的名词或代词，在从句中所起作用如下：Is he the man who/that wants to see you 他就是想见你的人吗？（who/that在从句中作主语）He is the man whom/ that I saw yesterday.他就是我昨天见的那个人。（whom/that在从句中作宾语）②Whose 用来指人或物，(只用作定语, 若指物，它还可以同of which互换）, 例如：They rushed over to help the man whose car had broken down.那人车坏了，大家都跑过去帮忙。Please pass me the book whose (of which) cover is green.请递给我那本绿皮的书。

③which, that它们所代替的先行词是事物的名词或代词，在从句中可作主语、宾语等，例如：A prosperity which / that had never been seen before appears in the countryside.农村出现了前所未 有的繁荣。（which / that在从句中作主语）The package which / that you are carrying is about to come unwrapped.你拿的包快散了。(which / that 在从句中作宾语）2、关系副词引导的定语从句关系副词可代替的先行词是时间、地点或理由的名词，在从句中作状语。

①when, where,why关系副词when, where, why的含义相当于"介词+ which"结构，因此常常和"介词+ which"结构交替使用.There are occasions when (on which) one must yield.任何人都有不得不屈服的时候。Beijing is the place where (in which) I was born.北京是我的出生地。Is this the reason why (for which) he refused our offer 这就是他拒绝我们帮助他的理由吗？②that代替关系副词that可以用于表示时间、地点、方式、理由的名词后取代when, where, why和"介词+ which"引导的定语从 句，在非正式文体中that常被省略（正式文体中不可省略），例如：His father died the year (that / when / in which) he was born.他父亲在他出生那年逝世了。