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定语从句是英语学习中的重点，也是英语学习中的难点，要学好定语从句要了解以下几点：

首先，了解什么是定语从句，定语从句是一种用句子作定语，修饰名词或代词的主从复合句，通常直接跟在被修饰名词或代词之后，被修饰的名词或代词称为先行词。定语从句一般由关系代词或关系副词来引导，关系代词或关系副词一方面连接定语从句，另一方面其本身又在定语从句中充当某个句子成份。

eg：She is a girl who just can't say no. girl 为先行词，who just can't say no 为定语从句，关系词who 在整个定语从句中作主语。

常见的关系代词who(whom)指人、that 指人或物、whose，关系副词when、where以及why、how。关系代词在定语从句中可作主语、宾语、定语、表语，关系副词在定语从句中作状语。

eg： (1)He  laughs  best who laughs last.(who 在定语从句中作主语)（2）David is the boy whom /who you have talked to. (who/whom作定语从句的宾语)（3）This is the window whose glass is broken.(whose作定语)（4）Tell me the reason why you are late.(why 在定语从句中作状语)

到底用关系代词还是关系副词，就看定语从句中缺少什么，如果缺少主语、宾语、定语就用关系代词，如果缺少状语就用关系副词。

其次，应注意定语从句中的谓语动词一般应与先行词保持人称和数的一致。

eg：This is the only one of the students who is from Canada.

                             先行词          单数

定语从句中的动词时态应根据句意及上、下文而定，主句和从句可用不同的时态。

eg:Where is the comrade whom I saw yesterday.

        现在时                过去时

第三，定语从句，分为限制性定语从句和非限制性定语从句。限制性定语从句是先行词在意义上不可缺少的定语，关系十分密切，如果省去主句的意思就不完整，该从句不能用逗号和主句分开。

eg:That is a man who found my handbag.

非限制性定语从句是先行词和整个主句的附加说明，关系不是十分密切，若省去，主句的意思仍然清楚，该从句前常用逗号和主句分开。

eg：She was a good mother, who loved her children very much.

需要注意引导非限制性定语从句指事或物时只用which。

eg：The trees near the house, which I planted three years ago, are growing up well.

第四，运用定语从句中的关系代词或关系副词要注意以下几点：

1.介词＋关系词（即关系代词在定语从句中作介词的宾语，如果介词放在关系代词的前面，则关系代词指人只能用whom，指事或物只能用which。）

who is the comrade with whom you shook hands just now?

The boy lives in the house in front of which there is a tall building.

当然，当介词放在从句的句末时，关系代词的用法不受以上限制。

2.关系代词that 可指人也可指物，但在下列情况下一般用that 引导，不用which/who。

1)当先行词为nothing, anything, everything, all, much, little, none, few，等不定代词时常用that，但something可用which。

eg：I can give you all that I can get.

2)当先行词被only, last 等修饰时常用that。

eg：This is the only picture that is inter esting.

3)当先行词被数词、形容词最高级来修饰时常用that，不用which。

eg：That is the longest river that runs through our village.

4)主句中有两个先行词，且一个表示人，另一个表示物，即先行词同时有人或物时常用that。

eg：We talked about the persons and the things that had never happened in school.

                           人          物

关系代词本身在定语从句中用作表语，用that引导定语从句，但that 常省略。

eg:Our  hometown is no longer the place that it used to be.

6)若句中有两个定语从句，前一个定语从句已用了关系代词which，则第二个定语从句中的关系代词常用that。

eg： Edison set up a factory which produced things that had never seen before.