1. 定义：用来修饰某个名词、代词或整个句子的从句就叫做定语从句。如：

The man you talked about is our headmaster. 你们刚才所谈论的那个人是我们的校长

This is the bike my father gave me as my birthday present. 这是我父亲送给我作为生日礼物的单车。

The tree whose leaves are red was planted last year. 那棵叶子红色的树是去年栽的

2. 先行词：定语从句所修饰的词就叫做先行词，它一般是由名词、代词或句子充当。如：

The boy who is singing at the stage is only nine years old. 在舞台上唱歌的那个男孩今年才九岁。(现行词是名词)

Anyone that is willing to help others is welcome to our group. 任何愿意帮助他人的人都欢迎来参加我们的组织。(现行词是代词)

His mother is out of danger, which excites their family very much. 他母亲脱离了危险，这件事情使他们家兴奋不已。(现行词是句子)

3. 关系词：引导定语从句的词就叫做关系词。根据关系词的用法不同，关系词又分关系代词和关系副词，其中关系代词有that, which, who, whom, whose, as, but等，关系副词有where, when, why等。关系词既起连接先行词与定语从句的作用，又做从句中的某个句子成分。如：

Lee is one of the students who are working very hard. 李是一个学习非常努力的学生。(引导词who)

There is nobody but knows you here. 这儿没有不认识你的人。(引导词but)

He led us to a place where we have never been before. 他把我们带到了我们以前从未去过的地方。 (引导词where)