定语从句在句中做定语，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。
关系代词有：who, whom, whose, that, which等。
关系副词有：when, where, why等。

一、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  指人，在定语从句中作主语。
    The boys who are playing football are from Class One. 正在踢足球的男孩是一班的。
    Those who want to go to the museum must be at the school gate at 7 tomorrow morning. 想去博物馆的人必须在明晨7点到大门口集合。
2．whom  指人，在定语从句中做宾语，常可省略。
    Mr Liu is the person ( whom ) you talked about on the bus.  刘先生就是你们在公共汽车上谈论的那个人。
    Li Ming is just the boy ( whom ) I want to see. 李明正是我想要见的男孩。
    注意：关系代词whom 在口语或非正式文体中常可用who 来代替，也可省略。
    The man ( whom / who )you met just now is my old friend.
3．Which 指物，在定语从句中做主语或宾语，做宾语时常可省略。
    Football is a game which is liked by most boys. 足球是大多数男孩所喜欢的运动。
    The factory which makes computers is far away from here. 制造计算机的那家公司离这儿很远。
    He likes to read books which are written by foreign writers. 他喜欢外国作家写的书。
    The house which is by the lake looks nice.  湖边的那幢房子看上去很漂亮。
    This is the pen ( which ) he bought yesterday.  这是他昨天买的钢笔。
4．That 指人时，相当于who 或whom；指物时，相当于which.。在定语从句中作主语或宾语，作宾语时常可省略。
    The number of people that / who come to visit this city each year reaches one million. 每年来参观这座城市的人数达一百万。
    Where is the man that / whom I saw this morning? 我今天早上看到的那个人在哪儿？
    The person that /whom you introduced to me is very kind. 你介绍给我的那个人很友好。
    The season that / which comes after spring is summer. 春天以后的季节是夏季。
    Yesterday I received a letter that / which came from Australia. 昨天我收到了一封来自澳大利亚的信。
5．Whose  通常指人，也可指物，在定语从句中做定语。
     I visited a scientist whose name is known all over the country. 我拜访了一个全国知名的科学家。
     He has a friend whose father is a doctor. 他有一个爸爸当医生的朋友。
     I once lived in the house whose roof has fallen in.  我曾经住在那幢屋顶已经倒塌了的房子里。
     注意：指物时，常用下列结构来代替：
     The classroom whose door is broken will soon be repaired.
     The classroom the door of which is broken will soon be repaired.