定语从句常考知识点
定语从句是主要用来修饰它前面的先行词（名词或代词）的从句，所以又称形容词从句。根据与先行词的关系，定语从句可分为限制性定语从句和非限制性定语从句。定语从句主要考查引导定语从句的关系代词或关系副词；介词＋关系代词以及非限制性定语从句。具体考点如下：

考点一：
引导定语从句的引导词有关系代词（who, whom, which, that, whose）和关系副词(as, when, where, why)等。但是当下列情况出现时，对引导词有特殊要求。
1.指物只能用 that不用which 的情况
 1).当先行词是不定代词时,如: all, little, much, none, one, everyone, something, anything, everything, nothing 等。
 2).当先行词被the only, the very, the same, little, few, no, 等修饰时。
 3).当先行词被序数词或形容词最高级修饰时。
 4).在疑问词who、which、what开头的句子中。
5)当定语从句为there be 句型时，关系代词只能用that,也可以省略。
This is the fastest train (that) there is to Beijing.
6).当主句是以which 或 who 开头的特殊疑问句时，关系代词只能用 that。
Who is the girl that is talking with Mr. Brown？
7).如果出现两个或两个以上的先行词，并同时兼指人和物时，关系代词用that。
We are talking about the people and countries that we have visited.
8).当先行词在定语从句中做表语时，关系代词只能用that,也可以省略。如：
China is not the country (that) it was.
 
2.指物只能用which 不用that的情况 
1).在非限制性定语从句中   
2).在介词后面
3.指人时只能用who不用 that的情况
1).先行词为one, ones或anyone. 
2).先行词为those. 
3).在there be开头的句子中。 
Eg: There is a man who called himself Mr. Wu at the gate.      
4). 在非限制性定语从句中。
4.“the same ...as”, “such...as” 中的as 可以指人或物，作从句的主语、宾语、表语或介词的宾语。如：
Such money as he earned was spent on spirits and tobacco.
Is this the same mosquito as bit you just now?
He was not half such a coward as we took him for.
5.why引导表示原因的定语从句，其先行词一般是reason。当主句主语是reason时，作主句表语的成分不能有because和because of 。其结构一般为the reason why…is that…，或者 the reason that … is that…，如：
　 He did not tell us the reason why he was late again.
The reason why （that）he didn't come is that he was ill.
 
考点二：
在“介词+which / whom”的结构中，介词的选择取决于三种情况：
1.定语从句中谓语动词或表语的搭配
　　This is the college in which I am studying.
　　He is the man about whom we are talking.
2.先行词与介词的习惯搭配
当先行词表示“领域、方面”时，如：aspect, respect, area, field等，用 in which；
当先行词表示“价格、利率、速度”时，如：rate, price, speed 等，用at which；
当先行词表示“程度”时，如：degree, extent等，用 to which；
当先行词表示“根据、依据、基础”时，如：grounds, foundation, basis等，用 on which；
The speed at which the machine operates is shown on the meter. 　仪表上显示出这台机器的运转的速度。
It is useful to be able to predict the extent to which a price change will affect supply and demand.
3.当定语从句为最高级时只能用of which，否则用其他介词
I have five dictionaries of which Longman Dictionary is the best.
I have five dictionaries among which Longman Dictionary is published in UK. 
注意：不可拆分的动词短语介词不能提前，如put up with；有的短语可能有不同的介词搭配，此时需要根据上下文来确定。如：be familiar with / to ; compare with / to。
 
考点三：
当先行词为表示时间的名词（如：time, day, year, week, month, occasion）或地点的名词（如：place, room, city, country）时，一般用关系副词 when 或 where. 分别相当于in which, at which, on which。但是当这些表示时间或地点的名词作从句的宾语时，则要用关系代词that 或which。
I can't forget the days when (in which) I lived with you.
Can you tell me the day when (on which) the first satellite was sent into space?
China is the only country where (in which) wild pandas can be found.
I’ll never forget the village where I spent my childhood.
I will never forget the days that (which) I spent in Beijing University.
　　(本句days 作spend 的宾语，故用关系代词that / which )
I have never been to Beijing, but it’s the place that I most want to visit.(本句place 作visit 的宾语。)
 
考点四：
非限定性定语从句不能用that引导, 一般用引导词which / as或who（指人），非限定性定语从句既可以修饰主句的部分内容，也可修饰主句的全部内容。as引导的非限定性定语从句既可以放在主句之前，也可以放在主句之后，但which或who引导的非限定性定语从句不能放在主句句首。
As we all know, the moon is a satellite of the earth.
The large area is covered with thick snow, which affects people's life greatly.
His speech, which bored us to death, was over at last.
Her sister, who lived in another city, was coming to visit her.
关于非限制性定语从句，应该注意：
1．非限制性定语从句和单句的比较 
I am reading Harry Porter, which is an interesting book.
He failed in the exam.   That made his parents angry.   
He failed in the exam, which made his parents angry.   
He has two sons.     Both of them are teachers.
He has two sons, both of whom are teachers.
2．as 引导的非限制性定语从句与it。
The earth is round.   It is known to all.   
The earth is round, which is known to all.                   
As is known to all, the earth is round.   
It is known to all that the earth is round. 
as 具有“正如”之意，搭配的动词一般是固定的，如：as you know/ as you see/as we planned/as we expected/as is reported...
 3.as 引导的限制性定语从句与其他从句的比较  
  This is the same book as I lost.这本书和我丢的那本一模一样。(定语从句)    
This is the same book that I lost.这本书就是我丢的那本。(定语从句)     
This is such an interesting book as we all like./ This is so interesting a book as we all like.  这是大家都喜欢的如此有趣的一本书。(定语从句)
 This is such an interesting book that we all like it. / This is so interesting a book that we all like it.  这本书如此有趣，大家都喜欢。(结果状语从句)

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