在复合句中修饰名词或代词的从句是定语从句。被定语从句修饰的名词或代词叫先行词，定语从句必须放在先行词的后面。

　　定语从句的原理揭秘：

　　the woman-she lives next door-is a lawyer.

　　The woman who lives next door is a lawyer.

　　更多的例子：

　　She is the girl. She met me yesterday.

　　She is the girl who met me yesterday.

　　She is the girl. I met her yesterday.

　　She is the girl who(whom) I met yesterday.

　　This is the book. I bought it yesterday.

　　This is the book which I bought yesterday.

　　定语从句一般由关系代词that, which, who, whom, whose 引导；或

　　关系副词 when, where, why 引导

　　作用如下：

　　（1） 把主句和从句连接起来；

　　（2） 在从句中但当一定的句子成分。

　　定语从句的分类

　　（1）限定性定语从句

　　限定性定语从句把其先行词限定于某一个特殊形态，去掉后影响整句的意思。

　　如：Life that has no goal is miserable.

　　* 在口语中，90%的定语从句都属于限定性定语从句。

　　(2 ) 非限定性定语从句

　　非限定性定语从句用来补充说明先行词，去掉后不影响整句的意思。

　　The youth need to have dreams, which are important.

　　限定性和非限定性定语从句的比较：

　　（a）I want a man who understands English.

　　（b）I will engage Mr. Smith, who understands English.

　　(a)句是限定性定语从句，表示我需要的是哪样一种人。（b）句是非限定性定语从句，只提供一些有关史密斯先生的事情。 如果去掉 （a）句的关系从句 who understands English 就会影响全句意义的完整；去掉(b)句的关系从句则无大碍。

　　（c）He has two sisters, who work in the Ministry of Education.

　　（d）He has two sisters who work in the Ministry of Education.

　　(b)句是非限定性定语从句，表示他只有两个妹妹。（d）句是限定性定语从句，表示他至少还有一个姐妹在别处工作。（一共至少三个姐妹）

　　（e）My father who is now in Sydney will return to Beijing soon. (误)

　　（f）My father, who is now in Sydney, will return to Beijing soon. (正)

　　（e）句是限定性从句，表示“我在悉尼的爸爸”句意不合理。（f）句是非限定性定语从句，“他现在在悉尼”只是对这句话的主题加以补充说明。

　　限定性定语从句与非限定性定语从句的区别

　　限定性定语从句非限定性定语从句

　　1. 关系代词之前没有逗号1. 关系代词之前有逗号

　　2. 做宾格的关系代词可省略2. 关系代词不可省略

　　3. 可用that 3. 不能用that

　　As 也可以引导定语从句！！As 本为连词， 但为省略其后的代词，它本身兼有代词的作用而成为关系代词。意思是正如。。。一般用在非限定性定语从句中：

　　As has been said,

　　As has been discussed previously,

　　As is widely accepted,

　　As is shown in the pie chart,

　　As和which都能引导定语从句，那么它们的共同点和区别是什么呢?

　　（1） As 从句可以放在句首；which 则不能放在句首。

　　（2） 当关系代词是一般动词时，只能用which；As 只能做Be动词的主语。

　　He saw the girl, as delighted him. ×

　　He saw the girl, which delighted him. √

　　He married her, as was natural.√

　　He married her, which was natural. √

　　关系代词与介词

　　关系代词做介词的宾语的时候，介词可置于whom和which 之前或之后，但关系代词为that时则仅能将介词置于其后。

　　This is the girl of whom I spoke.=This is the girl whom I spoke of.

　　This is the girl that I spoke of.√

　　This is the girl of that I spoke. ×

　　介词during，beyond, except 或表示部分的of, 通常置于关系代词之前：

　　The years during which he was away were long years to her.

　　He wrote many books, some of which you will read some day.

　　根据介词与先行词的搭配习惯：

　　Do you still remember the day on which we met for the first time?

　　根据定语从句中谓语动词与介词的搭配习惯确定介词:

　　This is the ring on which she spent 1,000 dollars.

　　雅思考官如何使用定语从句（8分-8.5分）：

　　Canada is the only country in which the numbers in prison have decreased over the period 1930-1980.

　　This is most evident in photographic film, where Germany spends much less than Britain.

　　However, the parents who do not allow their children sufficient free time for leisure activities outside school hours, are misguided.

　　If everything they do must have some educational or academic relevance, then they will soon get tired of studying altogether, which is the last thing parents would want.

　　These are the two factors that may be the most important for achieving happiness.

　　They are generally more confident and independent than children who stay at home with their parents and who are not used to strangers or new situations.