定语从句是中学英语语法中的重点内容，也是高考中常考查的一个考点。掌握定语从句的用法，必须突破以下六道难关：
一、选准关系代词和关系副词
在定语从句中，关系代词或关系副词都代替先行词在从句中充当一定的成分，如果该从句中缺少主语、表语、宾语或定语，就要用关系代词来连接从句；如果从句中缺少状语，就得用关系副词来连接从句。例如：
(1)I still remember the days which / that we spent together.
(2)I still remember the days when we worked together.
二、了解仅用that 引导限制性定语从句的几种情况
that 在限制性定语从句中既可指人又可指事或物。在下列情况下，只能用that 来引导定语从句：
1. 先行词是all , much, few, little, everything, anything, nothing 等不定代词时。例如：
Is there anything that you want to buy?
2. 先行词被all, few, little, much, every, some, no等词修饰或被the only, the very, the same, the last 等限定词修饰时。例如：
These are all the pictures that I have seen.
This is the very dictionary that is of great help.
3. 先行词既指人又指物时。例如：
My father and his teacher talked a lot about the things and the persons that they could remember.
4. 关系代词在定语从句中作表语时。例如：
Our school is not the one that it used to be.
5. 先行词被形容词最高级或序数词修饰时。例如：
This is the best English film that I have ever seen.
The first English novel that I read was A Tale of Two Cities.
6. 主句是who 或which 引导的特殊问句时。例如：
Who is the girl that is standing under the tree?
Which is the machine that we used last Sunday?
三、把握“介词＋关系代词”结构中介词的选用
介词的选择与主句中先行词的搭配密切相关。例如：
That”s the reason for which he was late for school.
This is the book on which I spent 8 yuan.
注意：
1. 定语从句中谓语动词是含有介词的动词短语时，介词应放在动词之后，不能拆开放在关系代词前。例如：
This is the key which you are looking for.
This is the baby whom you will look after.
2. 该结构中，关系代词指人时用whom，指物时用which.
四、弄清as 和which 引导的非限制性定语从句指代整体时的区别
1. 位置上的区别：as 引导的非限制性定语从句的位置比较灵活，可以放在主句前，也可以放在主句后，还可插在主句的中间；而which 引导的非限制性定语从句只能放在主句之后。例如：
As is known to us all, the earth turns around the sun.
The earth, as is known to us all, turns around the sun.
The earth turns around the sun, as is known to us all.
The earth turns around the sun, which our parents once told us when we were very young.
2. 词义与联系上的区别：as 引导的非限制性定语从句与主句之间有着较为密切的上下文联系，as 本身含有“正如”之意；而which 引导的非限制性定语从句与主句之间在逻辑意义上近似并列句，which 本身表示“这”或“这一点”之意。例如：
Light travels faster than sound, as we all know.
Jack came late for school, which made his teacher angry.
五、注意定语从句的主谓一致性
关系代词在定语从句中作主语时，从句的谓语必须与先行词保持人称和数的一致。例如：
Tom is one of the boys who are from the USA.
Tom is the only one of the boys who is from the USA.
六、保持主句的完整性并避免从句成分的重叠
1. 在定语从句中，不管出现任何情况，主句的结构必须完整，即主句必须有主语和谓语，缺一不可，否则，整个主从复合句就无法成立。试比较：
(1)Is this factory _______ we visited last week?
(2)Is this the factory _______ we visited last week?
A. which B. where C. what D. the one
分析：
第一题缺少表语，只有填D项才能保持主句的完整性，而其他的选项均为干扰项。
第二题主句本身完整，应填的关系代词在从句中作visited 的宾语，故应选A项。
2. 从定语从句中，关系代词或关系副词已经代替先行词独立地或与相应的介词一起在定语从句中充当了一定的成分，因此在定语从句中就不能再出现与关系代(副)词重叠的成分。例如：
错句：Is this the train that they took it last Sunday?
分析：该题中that 已经代替先行词the train 在定语从句中充当及物动词took 的宾语，故重叠出现的宾语it必须删去。
总之，突破了以上六道难关，定语从句中的难题便会迎刃而解。
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