被修饰的词叫先行词。定语从句不同于单词作定语的情况，它需要放在被修饰的词(即先行词)之后。定语从句一般由关系代词来引导。关系代词必须放在定语从句之首。定语从句（Attributive Clauses）在句中做定语，修饰一个名词或代词，被修饰的名词,词组或代词即先行词。定语从句通常出现在先行词之后，由关系词（关系代词或关系副词）引出。
关系代词引导的定语从句
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.农村出现了前所未有的繁荣。（which / that在句中作主语）The package (which / that) you are carrying is about to come unwrapped.你拿的包快散了。（which / that在句中作宾语）
关系副词引导的定语从句
1、when, where, why关系副词when巳呀屋饔, where, why的含义相当于"介词+ which"结构，因此常常和"介词+ which"结籁缰汾秘构交替使用，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，在口傥态尬锪语中that常被省略，例如：His father died the year (that / when / on 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。
判断关系代词与关系副词
1、方法一： 用关系代词，还是关系副词完全取决于从句中的谓语动词。及物动词后面无宾语，就必须要求用关系代词;而不及物动词则要求用关系副词。
2、方法二： 准确判断先行词在定语从句中的成分(主、谓、宾、定、状)，也能正确选择出关系代词/关系副词。
