定语从句的用法讲解
定语从句是高中英语的重要语法项目之一，它的结构和用法比较复杂。我们应从哪些方面有效地复习定语从句呢？下面就是小编给大家带来的定语从句的用法讲解，希望大家喜欢！
[定语从句的种类]
定语从句分为限制性和非限制性两种。限制性定语从句是句子不可缺少的部分，如去掉主句意思就不完整或失去意义；非限制性定语从句是先行词的附加说明，去掉也不会影响主句的意思。它与主句之间通常用逗号分开，一般不用that引导，关系词不可省略。如：
This is the book that I want to read. （限制性）
Tom’s father， who arrived just now， is a famous scientist. （非限制性）
非限制性定语从句的先行词成分可以是整个主句，非限制性定语从句常用which引导，修饰整个句子。定语从句的谓语动词要用第三人称单数。例如：
The weather turned out to be very good， which was more than we could expect.
关系代词引导的定语从句
关系代词所指代的先行词是表示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。
在定语从句中，who和that既可作主语又可作宾语，whom只可作宾语（作宾语时可以省略）。如：
Is he the man who/that wants to see you？ （who/that在从句中作主语）
The man （whom/who/that） you spoke to just now is our English teacher. （whom/who/that在从句中作宾语，可省略）
[whose]
在定语从句中作定语，可用来指人或物，表示物时相当于“the+名词+of+which”或“of+which+the+名词”。如：John， whose wife is ill， can’t come to the party.
[which & that]
which和that指代的先行词是表示事物的名词或代词，在定语从句中可作主语、宾语等。如：
Please show me the radio that/which you have just bought. （that/which在从句中作宾语，可省略）
This is the film that/which created a great sensation. （that/which在从句中作主语）
只能用that而不能用which的情况：
1. 先行词是最高级或被最高级修饰时。如：This is the best dictionary that I’ve ever used.
2. 先行词是序数词或被序数词修饰时。如：The first thing that we should do is to get some food.
3. 当先行词是anything， everything， nothing， something， few， all， none， little， some等不定代词时，或是由every， any， all， some， no， little， few， much等词修饰时。如：Tom told his father all that had happened.
There is little that I can do for you.
4. 先行词既有人又有物时。如：They talked about the teachers and the schools that they knew.
5. 先行词被the only，the very，the right等修饰时。如：This is the very book that I want to have.
6. 在which，who，whom引起的问句中，为避免重复。如：Who is the girl that talked to you yesterday？
7. 关系代词在定语从句中作表语时。如：China is not the country that it was.
只能用which，不能用that的情况：
1. 在非限制性定语从句中。如：Beijing， which is the capital of China， has a very long histroy.
2. 在介词后面，即“介词+which（指物）”。如：This is the house in which we lived three years ago.
[as]
关系代词as代替的先行词是人或物的名词， 在从句中作主语、宾语和表语。
as引导限制性定语从句时，常和such， the same， as（so）连用， 构成such ... as， the same ... as， as/so ... as等结构，在从句中可作主语、宾语和表语。如：
Don’t trust such men as praise you to your face. （在定语从句中作主语）
I have the same trouble as you have. （在定语从句作宾语）
It’s as pleasant a film as I have ever seen. （在定语从句作宾语）
但是当先行词被the same修饰时，that也可以引导定语从句，但意义有所不同。the same ... as（指同样或同类的）， the same ... that（指同一个）。例如：
This is the same watch as I lost.
This is the same watch that I lost.
as也可单独使用，引导非限制性定语从句，修饰整个主句或主句中的一部分，放在句首、句中或句末。如：
As is known to us all， smoking is harmful to one’s health. （指代整个主句，在从句中做主语）
He is late， as is often the case. （指代整个主句，在从句中做主语）
Light， as everybody knows， travels much faster than sound. （指代整个主句，在从句中做宾语）
as和which作关系代词的用法比较：
which引导的非限定性定语从句只能放在主句之后，which指代前面的一个句子或主句中的某一成分；as引导的非限定性定语从句能放在主句之前、中、后，既可指前面已经提到的情况，也可指后面提到的情况。
as引导从句时，有“为人所熟知”的含义，which则没有这层意思。as引导的从句往往表示的是一种附加说明，which引导的从句一般是较为重要的陈述。如：
His mother is ill， which makes him very sad.
As is expected， the England team won the football match.
[关系副词引导的定语从句]
关系副词when， where， why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， 相当于“介词+which”结构。例如：
I’ll never forget the time when/during which we worked in the factory.
This is the village where /in which he was born.
I don’t know the reason why/for which she looks unhappy today.
[判定用关系代词还是用关系副词]
关系代词与关系副词的选用，应看先行词在定语从句中充当什么成分：若作主语、宾语或定语，则用关系代词who，whom，that，which，whose，as等；若作状语， 就用where， when， why等关系副词。如：
The reason why I’m writing is to tell you about a party on Sunday. （状语）
The reason that/which he gave us sounded reasonable. （宾语）
Pisa is a city， which has a leaning tower. （主语）
Pisa is a city， where there is a leaning tower. （状语）
[介词+关系代词]
当关系代词作介词宾语时，就出现了“介词+关系代词”的结构引导定语从句的现象。当先行词指物时用“介词+which”，指人时用“介词+whom”，且关系代词不能省略。而这种结构中较难解决的问题是介词的选择问题：
1. 介词的确定可以根据整个句意来确定。
Oxygen is a kind of gas， without which we couldn’t live.
2. 介词可以根据与先行词的搭配来确定。
He gave me the book for which he paid a lot of money. （动词与介词搭配）
1949 was the year in which the P.R.C. was founded. （名词与介词搭配）
The snake， of which she used to be afraid， is her favorite animal now. （形容词与介词搭配）
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