关系词是在英语中引导定语从句的词。它分为关系副词和关系代词。关系词作为英语语法的一个重要组成部分，在英语教育中有很重要的位置。一般中国的学校在初中会教授关系词的知识。

关系副词

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)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 mustyield. 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 ouroffer? 这就是他拒绝我们帮助他的理由吗?

2)that代替关系副词，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。

关系代词

基本关系代词

1)关系代词有that, which, who, whom, whose, as等，绝对没有what。

who, whom, that

这些词代替的先行词是人的名词或代词，who做主语指人，whom作宾语指人，that既可作主语又可作宾语(作宾语可以省略)，可以指人也可以指物。在从句中所起作用如下：

(1) 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)

(2) 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)

Whose 用来指人或物

(只用作定语, 若指物，它还可以同of which互换，指人的时候也可以用of whom 代替)

(1) 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。

(2) Please pass me the book whose color (the color of which) is green. 请递给我那本绿皮的书。

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，作宾语时可以省略，例如：

(1) 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)

(2) The package (which / that) you are carrying is about to come unwrapped. 你拿那个包裹快要散开了。(which / that在句中作宾语)