回答这个问题前我们先看下定语从句的定义
• 定语从句的定义
定语从句在句子中作定语,用来修饰一个名词、名词词组或者代词.
• 先行词和引导词
被修饰的名词、名词词组或代词叫做先行词；
在先行词和定语从句之间起连接作用的词叫做引导词.
引导词分为“关系代词”和“关系副词”.
• 关系代词和关系副词
关系代词有：who,whom,whose,that,which,as.
关系副词有：when,where,why.注意：关系副词里面没有how.
如果要修饰方式,用that或in which引导,或者不用引导词.
I don’t like the way (that,in which) he eyed me.
我不喜欢他看我的那个样子.
• 关系代词：who
关系动词who在从句中主要作主语,在非正式语体里who还可以作从句中的宾语.
He is the man who wants to see you.
He is the man who I saw in the park yesterday.
• 关系代词：whom
He is the man （whom） I saw in the park yesterday.
（whom在从句中作宾语）
• 关系代词：whose
whose 用来指人或物,（只能用作定语,若指物,它还可以同of which互换）.
They rushed over to help the man whose car had broken down.
Please pass me the book whose cover is green.（of which the cover/the cover of which）
• 关系代词：which（1）
which指物,在从句中作主语或宾语,作宾语时在非正式语体中可以省略.
They needed a plant which didn't need as much water as rice.
The farm (which) we visited yesterday is located in the suburb of Beijing.
• 关系代词：which（2）
当在which和that面前进行选择的情况下,一下情况要选which：
1.在非限制性定语从句中通常用which作引导词,而不能用that做非限制性定语从句的引导词.
2.修饰整个主句.
I never met Julia again after that,which was a pity.
3.修饰谓语部分.
He can swim in the river,which I cannot.
4.介词 + which
They are all questions to which there are no answers.
• 关系代词：that（1）
that多用来指物,有时也可以用来指人；在从句中作主语或宾语.指物的时候多用that,也可用which.
It’s a question that (which) needs careful consideration.
（指物,作主语.）
Who is the man that is reading a magazine under the tree?
（指人,作主语.）
The girl (that) we saw yesterday is Tom’s sister.
（指人,作宾语,可省略.）
• 关系代词：that（2）
在以下的情况中,只能用that作引导词,而不能用which作引导词.
1.先行词为all ,everything,anything,nothing,little等不定代词时.
All (that) she lacked was training.
2.先行词被形容词最高级修饰时
This is the best film that I have ever seen.
3.先行词被序数词和the last修饰时
4.先行词中既有人又有物时
They talked about the teachers and the schools that they had visited.
5.主句是含有who或which的特殊疑问句,为了避免重复时
