定语从句
定语从句是由关系代词或关系副词引导的从句，其作用是作定语修饰主句的某个名词性成分，相当于形容词，所以又称为形容词性从句，一般紧跟在它所修饰的先行词后面。
关系词
引导定语从句的关联词称为关系词，关系词有关系代词和关系副词。关系代词有that, which, who, whom, whose, as等，绝对没有WHAT；关系副词有where, when, why等。关系词常有3个作用：
①连接作用，引导定语从句。
②代替主句中的先行词，甚至可能是主句中的一部分或者整个主句。
③在定语从句中充当一句子成分。
注：关系代词有主语、宾语之分。一般whom作为宾语。关系代词在从句中作主语，宾语，表语，定语等，关系副词在从句中作地点状语，时间状语，原因状语等。
定语
定语用来限定、修饰名词或代词的，是对名词或代词起修饰、限定作用的词、短语（动词不定式短语、动名词短语和分词短语）或句子，汉语中常用‘……的’表示。主要由形容词担任，此外，名词，代词，数词，分词，副词，不定式以及介词短语也可以来担任，也可以由一个句子来担任。单词作定语时通常放在它所修饰的词之前，作前置定语。短语和从句作定语时则放在所修饰的词之后，作后置定语。
先行词
被定语从句修饰的名词、代词称为先行词。
关系代词
关系代词所代替的先行词是人或物的名词或代词，并在从句中充当主语、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。
who指人在从句中做主语
(1) The boys who are playing football are from Class One. 在踢足球的男孩们是一班的.
(2) Yesterday I helped an old man who lost his way. 昨天，我帮助了一个迷路的老人.
whom指人
在定语从句中充当宾语，常省略。（注：who和whom已无太大区别，基本可以通用。唯一区别是who可以做主语而whom不可以。）
(1) Mr. Liu is the person (whom) you talked with on the bus. 刘先生就是在公交车上和你聊天的那个人.
(2) Mr. Ling is just the boy(whom)I want to see. 凌先生恰巧就是我想见的那个男孩.
(3) The man who/whom you met just now is my friend. 你刚刚见到的那个人就是我的朋友.
注意：关系代词whom在口语和非正式语体中常用who代替，可省略。
如果在从句中做宾语,就用whom或who.比如: He is the man whom/who I talk to. 他就是那个和我聊天的男人.
如果是在从句中作主语就只能用who.比如: He is the man who has an English book. 他就是那个有英语书的男人.
whose通常指人也可指物
在定语从句中做定语。
(1) He has a friend whose father is a doctor.
(2) I once lived in a house whose roof has fallen in.whose指物时，常用以下结构来代替
(3) The classroom whose door is broken will soon be repaired.
(4) The classroom the door of which is broken will soon be repaired.
(5) Do you like the book whose color is yellow?
(6) Do you like the book the color of which is yellow?
which指物
在定语从句中做主语或者宾语，做宾语时可省略。
(1) Football is a game which is liked by most boys. 足球是大多数男孩都喜欢的游戏.
(2) This is the pen (which) he bought yesterday. 这是他昨天刚买的钢笔.
that指人时
相当于who或者whom；指物时，相当于which。
在定语从句中做主语或者宾语，做宾语时可省略。
(5) The number of the people that/who come to visit the city each year rises one million.
(6) Where is the man that/whom I saw this morning?
关系副词引导的定语从句
when指时间
在定语从句中做时间状语
(1) I still remember the day when I first came to the school.
(2) The time when we got together finally came.
where指地点
在定语从句中做地点状语
(1) Shanghai is the city where I was born.
(2) The house where I lived ten years ago has been pulled down. why指原因
在定语从句中做原因状语
(1) Please tell me the reason why you missed the plane.
(2) I don’t know the reason why he looks unhappy today.
注意：关系副词引导的从句可以由“介词+关系代词”引导的从句替换
(1) The reason why/ for which he refused the invitation is not clear,
(2) From the year when/in which he was going to school he began to know what he wanted when he grew up.
限定性定语从句
关系代词
关系代词：在句中作主语、宾语或定语
1. that既可代表事物也可代表人，which代表事物；它们在从句中作主语或宾语，that在从句中作宾语时常可省略关系词，which在从句中作宾语也可以省略。[eg：This is the book （which）you want.]
