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。关系代词引导的定语从句关系代词所代替的先行词是人或物的名词或代词，并在句中充当主语、宾语、定语等成分。关系代词在定语从句中作主语时，从句谓语动词的人称和数要和先行词保持一致。1）who, 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）2) Whose 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。3）which, 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.   （which / that在句中作主语）     农村出现了前所未有的繁荣。The package which / that you are carrying is about to come unwrapped. (which / that 在句中作宾语）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。1）when, 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，例 如：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？2）that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，在口语中that常被省略，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。He is unlikely to find the place (that / where / in which) he lived forty years ago. 他不大可能找到他四十 年前居住过的地方。