定语从句修饰名词或代词，相当于一个较长的形容词。
The man who is talking to Mary is my teacher.
正在和 Mary 说话的那个人是我的老师。被修饰的名词叫先行词。
15.1 关系代词
| 关系代词 | 指代 | 在从句中作 |
|---|---|---|
| who | 人 | 主语/宾语 |
| whom | 人 | 宾语 |
| whose | 人/物 | 定语 |
| which | 物 | 主语/宾语 |
| that | 人/物 | 主语/宾语 |
例句：
The girl who helped me is my classmate.
帮助我的那个女孩是我同学。
The book which I bought yesterday is interesting.
我昨天买的那本书很有趣。
The man whose car was stolen called the police.
车被偷的那个男人报了警。15.2 关系副词
| 关系副词 | 表示 | 相当于 |
|---|---|---|
| when | 时间 | in/on/at which |
| where | 地点 | in/at which |
| why | 原因 | for which |
例句：
I still remember the day when we first met.
我仍然记得我们第一次见面的那一天。
This is the place where I grew up.
这是我长大的地方。
That is the reason why he left.
那就是他离开的原因。15.3 限制性与非限制性定语从句
限制性定语从句提供必要信息，不用逗号隔开：
Students who study hard usually make progress.
努力学习的学生通常会进步。非限制性定语从句提供补充信息，用逗号隔开：
My brother, who lives in Beijing, is a doctor.
我哥哥是一名医生，他住在北京。非限制性定语从句中通常不用 that。
错误：My brother, that lives in Beijing, is a doctor.
正确：My brother, who lives in Beijing, is a doctor.15.4 关系词省略
关系代词在从句中作宾语时，常可省略。
The book that I bought is useful.
The book I bought is useful.关系代词作主语时不能省略：
正确：The man who called you is my father.
错误：The man called you is my father. 若想表达“给你打电话的男人”注意：The man called you 更可能理解为“那个男人叫了你”，不是定语从句省略。