SAT语法中定语从句的用法
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 process of 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, 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
  (2)定语从句里面关系副词的用法。
  1)常用的关系副词：when, where, why
  2) When引导定语从句的用法。
  A. 先行词必须是表示时间的名词。
  Or so the thinking has gone since the early 1980s, when juries began holding more companies liable for their customers’ misfortunes.
  B. 如何区分when引导的定语从句和状语从句。用句子表示状语就是状语从句。例如：Whe
n I go to school, I will work hard.
  从两方面区分：
  When 引导的定语从句前一定有表示时间的名词;When 引导的状语语从句前没有有表示时间的名词。
  从翻译的角度看如果是引导的定语从句可以不翻译。如果是状语从句就必须翻译成当……时候。
  When=on which
  Where
resolve to do sth
  where=in which
  where引导的从句先行词必须是表示地点的名词。例如：
  I was born in Beijing where the Olympic games will be held.
  先行词表示地点，不一定用where来引导。例如：
  I have never been to Beijing, but it’s the place __.
  A. where I’d like to visit
  B. in which I’d like to visit
  C. I most want to visit
  D. that I want to visit it most
  3.区分关系代词引出的定语从句和关系副词引出的定语从句
  考试中涉及定语从句的考点经常是选关系词，而区分关系代词和关系副词是做题依据，关系代词中除whose在句中作定语外，其他各词均在句中作主语或宾语，即除去关系代词后，其后不是一个完整的句子;而关系副词在句中作状语，即除去关系副词后，其后是一个完整句子。
  例1：I’ve kept up a friendship with a girl whom I was at school with twenty years ago.(画线部分不是一个完整的句子)(1998年第8题)
  分析：该句是复合句，whom引导的定语从句修饰a girl，其中短语be with sb. at school，意为“在学校与某人是同学”。
  译文：我与一个二十年前和我在一起读书的女孩一直保持着友谊。
  例2：Besides, this is unlikely to produce the needed number of every kind of professional in a country as large as ours and where the economy is spread over so many states and involves so many international corporations.(画线部分是一个完整句子)(1999年Text 3)
  分析： 该句是复合句。主干部分为this is unlikely in a country，形容词短语as large as ours和定语从句where the economy is 都修饰country, is spread over和involves在定语从句中作并列谓语。
  译文：另外，像我们这样一个经济发展遍及很多地区，涉及很多跨国公司的国家，职业教育也不可能培养出各种职业所需要的专业人才。(小马过河国际教育)

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