一、定语从句的概念
定语从句是指在复合句中，修饰限定某一名词或代词的从句。
例如：You must buy me the house which is near Xiangshan.
上面例句中的the house就是后面定语从句所修饰或限定的词，叫先行词，定语从句放在先行词的后面。
二、关系词(连接词)
引导定语从句的关系词有关系代词：that, which, who (宾格whom, 所有格whose) 和关系副词where, when, why。关系词放在先行词和定语从句之间，起连接作用，同时还做定语从句的一个成分。
1. 关系代词引导的定语从句
引导定语从句的关系代词主要有that、which、who、whom、whose，其中whom引导的是宾格，whose引导的是所有格。
2.that与which的用法区分
1)只能用that，不能用which作为定语从句关系代词的情况
a)当先行词由不定代词anything, nothing, the one, all, much, few, any, little等代替时，关系代词只能用that而不能用which 。例如：There is nothing that I can do
b)先行词有the only, the very, the same, the last修饰时
例如：The last place that I visited was the hospital.
c)先行词为序数词、数词、形容词级。
例如：This is the first composition that he has written in English.
d)先行词既有人，又有物时
例如：He talked about the teachers and schools that he had visited.
2)只用which不用that的情况
a)引导非限定性定语从句时
例如：The tree, which is four hundred years old, is very famous here.
b)介词后用which
例如：We depend on the land from which we get our food.
3.关系副词引导的定语从句
关系副词可代替的先行词是时间、地点或理由的名词，在从句中作状语。
1)先行词是表示地点名词时(country, school, room…)，而关系词代替先行词在定语从句中充当地点状语，一般用where引导定语从句。也可以用介词+which的结构。值得留意的是which前的介词选择是根据先行词与介词的搭配关系而定。
例如：He wants to find the place in which (=where) he lived forty years ago.
2)先行词是表示时间名词时，而关系词代替先行词在定语从句中充当时间状语，一般用when引导定语从句。也可以用介词+which的结构。值得留意的是when前的介词选择是根据先行词与介词的搭配关系而定。
例如：I cannot forget the first day on which (=when) my family moved into the city.
3)先行词是the reason，而关系词代替先行词在定语从句中充当原因状语，一般用why引导定语从句。也可以用for + which的结构。
例如：Is this the reason for which (why) he refused our offer?
但是这里要指出的是，假如介词和Which搭配在一起并不表示以上三种意思，则我们只能留存介词+Which的形式。如 I know English in which I wrote the love letter.
4.判定关系代词与关系副词
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。及物动词后面无宾语，就必需要求用关系代词;而不及物动词则要求用关系副词。
方法二：正确判定先行词在定语从句中的成分(主、谓、宾、定、状)，也能准确选择出关系代词/ 关系副词。
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)。
5.限定性从句和非限定性从句
1)定语从句有限制性和非限制性两种。限制性定语从句是先行词不可缺少的部门，去掉它主句意思往往不明确;非限制性定语从句是先行词的附加说明，去掉了也不会影响主句的意思，它与主句之间通常用逗号分开。例如：
All the Greeks who are philosophers are very clever. (限制性)
All the Greeks, who are philosophers, are very clever. (非限制性)
2)非限制性定语从句还能将整个主句作为先行词, 对其进行润饰, 这时从句谓语动词要用第三人称单数。例如：The traffic of Beijing is quite bad, which is known to everyone.
6.定语从句的省略
(1) 关系代词充当从句宾语
我们前面说到连接词在定语从句中充当一个成分，有可能是主语、有可能是宾语，我相信大家肯定还记得，这里我们要告诉大家，当这些连接词充当的是宾语时，可以省略。例如：
1. The book (that/which) we bought last year is now very popular.
2. We can see the boys (who/whom/that) the teacher is talking with are quite happy.
(2) 关系代词充当从句主语
当关系代词充当从句主语时，也可以省略，但是省略后的从句要发生形式的变化，原从句若为主动语态，后面的动词结构要变成分词ing形式，原从句若为被动语态，后面的动词结构直接为过去动词结构。这种结构在英语语法中称为分词作定语。例如：
The novel which was given by the professor mainly discusses the development of Chinese economy.
The novel given by the professor mainly discusses the development of Chinese economy.
The novel which mainly discusses the development of Chinese economy was given by the professor.
The novel mainly discussing the development of Chinese economy was given by the professor.
SP: (1) why的先行词reason或可用that 指代的名词也是定语从句的原因状语时，关系代词或关系副词可以省略。例如：
1. That is the reason (for which/why) you dislike it.
2. I wonder the reason (why/for which) he changed his mind.
(2) 表示方式的先行词way，其后的关系代词可用that也可省略，例如：
1. Everyone does not like the way (that) he talks.
2. Do you know the way (that) the teacher treated his students.
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