导语：定语从句是英语从句中比较常考的知识点，下面小编详细讲解定语从句，欢迎参考。
1. 定语从句的概念和特点
定语从句在复合句中起定语作用，修饰主句中的名词或代词。被定语从句所修饰的词叫先行词。定语从句一般在先行词之后。引导定语从句的词叫关系词。关系词在先行词与定语从句之间起纽带作用。关系词代表着先行词并在定语从句中充当一个句子成分。如：
A man who sells books in a shop is called a bookseller. 开店售书的人叫做书商。(who sells books为定语从句，修饰先行词a man)
The discussion was on the election which had taken place the day before. 讨论的内容全是有关前一天选举的.事。(which had taken place the day before为定语从句，修饰先行词the election)
2. 关系代词和关系副词的用法
根据关系词在定语从句中的不同用法，可分为关系代词和关系副词。关系代词主要有who, whom, which, that, whose等，在定语从句中作主语、宾语、定语等；关系副词主要有when, where, why等，在定语从句中作状语。具体用法如下：
(1)关系代词的用法：当先行词指人时，若它在从句中作主语，则用who；若它在从句作宾语，则用whom或who；若它在从句中作定语，则用whose。如：
The man who told me this refused to tell me his name. 告诉我这事的人不肯告诉我他的名字。
The woman whose umbrella you took is very angry about it. 你拿了那个女人的伞，她非常生气。
当先行词指物时，若在定语从句中作主语或宾语，则用which；若它在定语从句中作定语用，则用of which或whose。如：
I saw something in the paper which might interest you. 我在报上看到一样可能使你感兴趣的东西。
He’s written a book the name of which (whose name) I’ve forgotten. 他写了一本书，书名我给忘了。
(2) 关系副词的用法：关系副词主要有when, where, why等，在定语从句中作状语，分别表示时间、在点和原因。when主要放在time, day, years, season, age, occasion等时间的名词后；where主要放在place, city, town, village, house等地点名词后；why则通常只放在reason后。如：
Wednesday is the day when the shops are not open in the afternoon. 星期三下午商店不开门。
Give me one reason why we should help you. 给我举出一个我们应当帮助你的理由。
要特别注意，activity, case, point, even, situation, job, organization, position等抽象名词后接where引导定语从句的用法(这是近几年高考的一个新热点)。如：
I don’t want a job where I’m chained to a desk all day. 我不想找一份整天坐办公桌前的工作。
Today, we’ll discuss a number of cases where beginners of English fail to use the language properly. 今天，我们将讨论一些英语初学者对英语使用不当的问题。
The treatment will continue until the patient reaches the point where he can walk correctly and safely. 治疗将会持续到病人可以正常且安全地行走。
If you risk something important, you cause it to be in a dangerous situation where you might lose it. 你如果拿重要的东西去冒险，那你就是将它置于一种可能会失去它的危险境地。
3. 关系代词that与which的用法区别
两者都可指物，常可互换。其区别主要在于：
(1) 通常要用which的场合：
①引导非限制性定语从句时，通常要用which。如：
She received an invitation from her boss, which came as a surprise. 她收到了老板的邀请，这是她意想不到的。
②直接放在介词后作宾语时，通常要用which。如：
The tool with which he is working is called a hammer. 他干活用的那个工具叫做锤子。
(2) 通常多用that的场合：
①当先行词是下列不定代词或被它们修饰时much, little, none, all, few, every(thing), any(thing), no(thing) 等时，通常用that：
All (Everything) that can be done must be done. 凡能做的事都必须做。
但在现代英语中，在something, anything, everything等之后用which引导定语从句的现象也很普通。
②当先行词有the very, the only, the same 等修饰时，通常用that。如：
This is the only example that I know. 我知道的例子只有这一个。
Those are the very words that he used. 那是他的原话。
③当先行词有形容词最高级或序数词(包括last, next等)等修饰时，通常用that。如：
This is the best dictionary that I’ve ever used. 这是我用过的最好的词典。
The first thing that you should do is to work out a plan. 你应该做的第一件事是订个计划。
④当关系代词在定语从句中用作表语时，通常用that。如：
China is not the country (that) it was. 中国已不是过去的中国了。
⑤当先行词是一个既指人又指物的并列词组时，通常用that。如：
They talked about the persons and things that most impressed them. 他们谈论了使他们印象最深的人和事。
⑥当要避免重复时。如：
Which is the course that we are to take? 我们选哪门课程？
