定语从句一直是历年考研的一个重点和难点，对于英语基础薄弱的学生而言，如何攻克成为一个棘手的问题。面对多数同学的困惑，小编将在本节着重讲解定语从句的一些知识，广大考生可以此为参考，一方面给自己增加信心，一方面让自己的英语知识更加扎实。
一、定语从句的定义：
用来修饰前面的名词或者代词的具有完整的主谓结构的句子就叫定语从句。定语从句有两个必备的基本元素。一个是先行词;一个是关系词。被定语从句所修饰的词称为先行词，定语从句必须置于先行词之后。
1. 先行词
(1)名词或代词做先行词。
例：He laughs best who laughs last.
(2)短语做先行词。
例：Many of life’s problems which were solved by asking family members, friends or
colleagues are beyond the capability of the extended family to resolve.
(3)句子做先行词。
例：The Greeks assumed that the structure of language had some connection with the processof thought, which took root in Europe long before people realized how diverse languages could
be.
I am a student, which you all know.
2. 关系词/引导词(关系代词、关系副词)
(1)定语从句里面关系代词的用法。
1) 常用的关系代词：which, that, who(m), whose, what, as, than。Whom现在用的很少。前面有介词时只能用which，不能用that。 定语从句的先行词为all, everything, something, nothing, anything或用作名词的little, much或先行词前有only, any, no时，先行词为序数词或形容词最高级所修饰时，先行词是两个不同性质的词，一个指人，一个指物时，先行词表示确切时间(time，day)、距离 (distance)、方向(direction)、倍数(times)、方式(way)时，关系代词用that。
2) 关系代词what, as, than在考试中的特殊情况。
A. What
a. what是子母同体。既是先行词也是关系词。所以，What前面不应该再出现先行词。 如果前面已经有一个先行词，就不能用what来引导。
例：You can have everything what you like. what=all that, 所以这儿不能用what.
b. what单独使用，后面不加名词。
例：She is not what she used to be.
c. what 后面加名词
例：What money I have has been given to you.
B. as
as引导定语从句。既可引导限制性定语从句，也可引导非限制性定语从句。引导限制性定语从句一般是固定用法。如as…as, such…as, the same...as考试中经常考到。
例：Such a student as works hard were be sure to succeed.
as引导非限制性定语从句。as引导的定语从句修饰整个句子时，一定要和主句用逗号隔开，即非限制性定语从句，先行词就是它所修饰的整个句子，该从句可放在主句前、主句后或插入主句中，as在句中作主语或宾语。
例：As is so often pointed out knowledge is a two edge weapon which can be used equally
for good or evil.
I was a boy which you know. which可以用as替换。
C. than 作为关系代词可以引导定语从句
例：More families consist of one parent households or two working parents; consequently,
children are likely to have less supervision at home ____ was common in the traditional family structure .
A. than B. that C. which D. as
特点：
1) than作为关系代词引导词在从句中一般必须做主语或者作宾语。
2) than 前面的主句必须要有比较级
3) 比较级所修饰的名词就是than指代的对象
