【摘 要】 主要说明定语从句中关系词的特殊用法，以及定语从句和同位语从句的区别。 【关键字】 定语从句 关系词 同位语从句 定语从句是诸多从句中重要而且用的频率最多的从句。但是，并不是每个学生都能轻而易举的把它掌握好的。而定语从句又是一个重点，如何才能解决这个问题呢？多年的教学实践总结出一些对掌握定语从句一些有益的方法：
一、关系词that的特殊用法
定语从句关系词的选用是至关重要的，也是学生难于掌握的，一般的用法都不在话下，但是，有些关系词的特殊用法就不是那么好掌握了。例如：关系词that，一般来说that在定语从句中运用最广既可以指人也可以指物，在从句中既可以做主语也可以做宾语。that的特殊用法如下：
1、当先行词为all、few、everything、nothing、nobody、everybody、everyone、little、much、anything、something、any等不定代词时只能用that如：Everybody that came here likes this buliding.There is nothing that I am interested in.
2、当先行词是续数词，或被形容词最高级修饰或被序数词修饰时，
This is the best book that I have bought .He is the first student that finished homework.
3、被the last、the only、the very、the small修饰时关系词多用that如：This is the last place that I want to visit。He is the only person that knows Jean"s address.
4、当先行词被all、little、any、every、no、much、none、some、any、just等不定代词时多用关系词that
5、当先行词同时含有人和物时只能用that.如：We talked about the things and persons that we were interested in The photograph often reminds her of the people and cities that he visited in Chinese.
6、不管指人或是指物，只能用that 的情况
(1)由which 开头的问句中。如;Which is the subject that you are going to learn next time?
(2)由who开头的问句中，也只能用thatWho is the man that came here?
二、关系副词的用法
定语从句中关系副词不多有when、why、where 但学生选关系词时总是不明白何时用关系副词何时用关系代词，下边就以例句加以说明。
误：Great changes have taken place in the shool where you visited last year.
正：Great changes have taken place in the school which you visited last year.
说明：在这两句中虽然先行词school是地点名词，但是关系词在从句中充当的是visited 的宾语，充当宾语的只能是关系代词而不能是关系副词。关系词的选择不能只看先行词是地点名词就选where，关键是要看关系词在定语从句充当的成分，如果关系词在定语从句中作主语或宾语就不能选关系副词where，如果先行词是地点名词而且关系词在定语从句中又充当状语就选关系副词where.
正：The reason why he missed the train was that he got up late.
误：The reason why he gave wasn"t true.
说明：虽然在这两句中先行词都是表原因的名词reason 但第二句中关系词在定语从句中充当的是gave的宾语所以不能选关系副词只能用关系代词。而第一句中关系词在定语从句中充当状语，所以就用关系副词。
关系副词when、where、when的含义相当于"介词+which:",因此常常和"介词+which"结构交替使用，例如：
There are occasions when(on which)one must yield.Beijing is the place where(in which)I was born. Is this the reason why(for which)he refused our offer?而且关系词在作介词宾语时不能省，同样在非限制性定语从句中也不可省去。
三、as引导的定语从句
定语从句中as作关系词不多也是学生不容易掌握的下面就具体说明，as既可以引导限制性定语从句，也可引导非限制性定语从句。但在引导两种定语从句时有所不同，as在引导限制性定语从句时，主要用在"such…as" 和"the same…as"的结构中，可以代替先行词是人或物的名称。Let"s discuss only such questions as concern us.(as作主语)I"ve never seen such a talented young as he is(as作表语)This is the same book as I read lasst week.(as作宾语)We drove out of the town by the same road as we had entered by .关系代词as 还可以用与as….as的结构中引出定语从句，例如：He smokes as expensive a cigarette as he can offord.(as作宾语)As many books as are in the school library have been made full use of.(as主语)as也可引导非限制性定语从句，但这时候as代替整个句子，对其进行说明。该从句的位置比较灵活，可位语主句之前，或插在主句中间，或位于主句后面。通常有逗号与主句隔开。
四、同位语从句与定语从句的区别
1.意义不同
定语从句对先行词进行修饰和限制，同位语从句对先行词进行解释或说明，指明先行词的具体内容，定语从句修饰的名词等于定语从句的关系词，同位语从句所修饰的名词等于整个同位语从句。试比较：
We are glad at the news that he will come.(new的内容就是that he will come 故that 引导的是同位语从句)
We are glad at the news that he told us.听到他告诉 我们的消息我们很高兴。(that 引导的从句对news修饰和限制故that引导的是定语从句)
2.引导词的功能上的不同
引导定语从句的that在从句中充当句子成分，而引导同位语从句的that在句子中不担任何成分，如：
The news that we have won in the football game aroused great excitement among the fans.(同位语从句，that在句子中不担任何成分)
The news that he told us is very exciting.(定语从句，that在从句中作told的宾语)
换言之，先行词在定语从句中有他的位置，但在同位语从句中就找不到他的位置。这是区别定语从句和同位语从句的关键点。
同位语从句有时可以脱离先行词而独立充当先行词在主句中的所充当的成分，定语从句则不能。试比较：
We all know the fact that they have modernized their factories.(同位语从句)
此句可写成
We all know that they have modernized their factories.
3.另外，两种从句修饰的名词范围不一样，同位语从句的先行词是抽象名词，而定语从句的先行词既可以是抽象名词也可以是表示具体事物的名词。希望以上的内容能给对定语从句有疑惑的学生起到一定的帮助。
参考文献
[1]孙志成，新英语教程，第二版，清华大学出版社
[2]章振邦，新编英语语法教程，上海外语教育出版社
[3]王自玲，全国英语等级考试，北京大学出版社