1.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。
2.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。
3.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。
4、先行词：
被定语从句修饰的名词、代词称为先行词。
[编辑本段]关系代词引导的定语从句举例
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。
1、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）
2、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 (of which) color is green. 请递给我那本绿皮的书。
3、 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. 你拿的包快散了。(which / that在句中作宾语） 4、 as as 可以做主语，也可以作宾语，作宾语时可以省略，指人或者物。
[编辑本段]限定性定语从句
一、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.] 2而且，如果which在从句中作“不及物动词＋介词”的介词的宾语，注意介词不要丢掉，而且介词总是放在关系代词which的前边，但有的则放在它原来的位置 3. 代表物时多用which，但在下列情况中用that而不用which： a)先行词是anything, everything, nothing , none等不定代词时； b)先行词由every, any, all, some, no, little, few, much等修饰时，这时的that常被省略； c)先行词前有序数词或形容词最高级修饰时； d)先行词就是序数词或最高级时； e)先行词中既有人又有物时； f)整个句中前面已有which时； g)当先行词为物并作表语时。 4. who和whom引导的从句用来修饰人，分别作从句中的主语和宾语，whom作宾语时，要注意它可以作动词的宾语也可以作介词的宾语 5. whose是关系代词，修饰名词作定语，相当于所修饰成分的前置所有格.它引导的从句可以修饰人和物, 当它引导的从句修饰物体时, 可以与 of which 调换,表达的意思一样。
二、关系副词（在句中作状语）
关系副词=介词+关系代词 why=for which where=in/ at/ on/ ... which (介词同先行词搭配) when=during/ on/ in/ ... which (介词同先行词搭配) 1. where是关系副词，用来表示地点的定语从句。 2. when引导定语从句表示时间〔注〕值得一提的是，表示时间“time"一词的定语从句只用when引导，有时不用任何关系代词，当然也不用that引导。 By the time you arrived in London, we had stayed there for two weeks. I still remember the first time I met her. Each time he goes to business trip, he brings a lot of living necessities, such as towels, soap, toothbrush etc。 3. 当从句的逻辑主语是some, any, no, somebody, anybody, nobody, something, anything, everything或nothing时，常用there is来引导 There is somebody here who wants to speak to you. 这里有人要和你说话。
[编辑本段]非限定性定语从句
非限定性定语从句的作用是对所修饰的成分作进一步说明，通常是引导词和先行词之间用逗号隔开，将从句拿掉后其他部分仍可成立 1. which引导的非限定性定语从句来说明前面整个句子的情况或主句的某一部分 2.当先行词是专有名词或物主代词和指示代词所修饰时，其后的定语从句通常是非限制性的，例如： Charles Smith, who was my former teacher, retired last year. 查理·史密斯去年退休了，他曾经是我的老师。 My house, which I bought last year, has got a lovely garden. 我去年买的的那幢房子带着个漂亮的花园。 This novel, which I have read three times, is very touching. 这本小说很动人，我已经读了三遍。 3.非限制性定语从句还能将整个主句作为先行词, 对其进行修饰, 这时从句谓语动词要用第三人称单数，例如： He seems not to have grasped what I meant, which greatly upsets me. 他似乎没抓住我的意思，这使我心烦。 Liquid water changes to vapor, which is called evaporation. 液态水变为蒸汽，这就叫做蒸发。 4. 有时as也可用作关系代词 5. 在非限定性定语从句中，不能用关系副词why和关系代词that，而用who, whom代表人，用which,whose代表事物，如果需要用why， 可用for which代替.； p.s: which引导非限定性定语从句其后不可省略成分，as可以
[编辑本段]关系代词引导的定语从句
1.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.
2. 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.
3. 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?
4.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.
5. 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?
[编辑本段]关系副词引导的定语从句
1. 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.
2. 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.
3. 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.
[编辑本段]介词和关系代词
1）介词后面的关系词不能省略。 2）that前不能有介词。 3） 某些在从句中充当时间，地点或原因状语的"介词+关系词"结构可以同关系副词when ,where和why 互换。 This is the house in which I lived two years ago. This is the house where I lived two years ago. Do you remember the day on which you joined our club? Do you remember the day when you joined our club? This is the reason why he came late. This is the reason for which he came late. 介词+关系代词引导的定语从句关系代词在定语从句中做介词宾语时 从句常由介词+关系代词引导 当介词放在关系代词前面时，关系代词只能用which/whom即“介词+which/whom"且不能省略。但当介词位于末尾时可用that/which/who/whom.作介词的宾语，且可以省略。例如： (1) The school (that/which) he once studied in is very famous."that/which"可以省略 = The school in which he once studied is very famous."which”不可省略 (2) Tomorrow I will bring here a magazine (that/which) you asked for. = Tomorrow I will bring here a magazine for which you asked. (3) We’ll go to hear the famous singer (whom/that/who) we have often talked about. = We’ll go to hear the famous singer about whom we have often talked. 注意： 1. 含有介词的动词短语一般不拆开使用，如：look for, look after, take care of等 T This is the watch which/that I am looking for. (T＝正确) F This is the watch for which I am looking. (F＝错误) 2. 若介词放在关系代词前，关系代词指人时用whom，不可用who或者that；指物时用which，不能用that；关系代词是所有格时用whose (1) The man with whom you talked is my friend. (T) The man with that you talked is my friend. (F) (2) The plane in which we flew to Canada is very comfortable. (T) The plane in that we flew in to Canada is very comfortable. (F) 3. “介词+关系代词”前可有some, any, none, both, all, neither, most, each, few等代词或者数词 (1) He loved his parents deeply, both of whom are very kind to him. (2) In the basket there are quite many apples, some of which have gone bad. (3) There are forty students in our class in all, most of whom are from big cities.
关系代词
关系代词（一般情况下）that 可用who ,whom 和which在从句做主语，谓动词的宾语，但是不能做介词的宾语。 which指物，在从句中作主语，谓语或宾语； who在从句中作主语； whom在从句中宾语； where在从句中修饰表地点的名词，做地点状语； when在从句中通常修饰表时间的名词，做时间状语； why在从句中做原因状语，先行词通常是"reason" 有时why也可用for+which代替。 例：A doctor is a person ‖who looks after people's health. 主语 谓语 先行词 定语从句修饰先行词
判断介词和关系代词
方法一： 用关系代词，还是关系副词完全取决于从句中的谓语动词。不及物动词后面无宾语，就必须要求用关系副词或者是介词加关系代词；而及物动词后接宾语，则要求用关系代词。例如： This is the mountain village where I stayed last year. I'll never forget the days when I worked together with you. 判断改错（注：先显示题，再显示答案，横线；用不同的颜色表示出。） （错） This is the mountain village where I visited last year. （错） I will never forget the days when I spent in the countryside. （对） This is the mountain village (which) I visited last year. （对） I'll never forget the days (which) I spent in the countryside. 习惯上总把表地点或时间的名词与关系副词 where, when联系在一起。此两题错在关系词的误用上。 方法二： 准确判断先行词在定语从句中的成分(主、谓、宾、定、状)，也能正确选择出关系代词/关系副词。 例1. Is this museum ___ you visited a few days ago? A. where B. that C. on which D. the one 例2. Is this the museum ____ the exhibition was held? A. where B. that C. on which D. the one 答案：例1 D，例2 A 例1变为肯定句： This museum is ___ you visited a few days ago. 例2变为肯定句： This is the museum ___ the exhibition was held. 在句1中，is后应跟表语，只有the one可以，而后面的you visited a few days ago则做one的定语从句。 而句2中, 主、谓、宾俱全，从句部分为句子的状语表地点，既可用副词where，又因 in the museum词组，可用介词in + which 引导地点状语。而此题中，介词on 用的不对，所以选A。 关系词的选择依据在从句中所做的成分，先行词在从句中做主、定、宾语时，选择关系代词 (who, whom, that, which, whose)； 先行词在从句中做状语时，应选择关系副词 ( where 地点状语，when 时间状语，why 原因状语) 。
参考资料：百度