定义：定语是在句子中承担定语功能，在咒语场上修饰某个名词或代词的句子。被定语从句修饰的名词、代词称为先行词，引导定语从句的词成为关系词。关系词连接先行词和定语从句，代替先行词在句子中形成一定的句子成分，有关系代词和关系副词两种。

1.关系代词：给予定语从句作为宾语、标语、定语常用的有who、whom、whose、that、which、as等。

(1)使用who/whom

两者都可以指人，但who在定语从句中给出宾语，whom仅用作宾语。

E.g . the man who was in the office yesterday is my father。/she is the person to whom you should address the request .

注意：如果whom不在介词后面，则可以用who替换，也可以省略，但在非限制性定语部分，不能用who替换whom。

e . g . do you know the girl(who/whom)they are talking about？

(2)whose的用法

可以指人，可以指东西，也可以在定语节说定语。

E.g . you are the only one whose advice she might listen to。/this house whose windows face the street is my aunt ' s .

注意：“whose名词”经常可以被“the名词of关系代词”或“of关系代词the名词”代替。

E.g . I will talk to those students whose homework/the homework of whom/of whom the homework hasn ' t been done .

(3)惠氏的用法

一般指事物，定语在从句中给予，形成宾语，偶尔说定语。

E.g . the river which runs through the centre of the city has been polluted seriously。/they stayed with me for three weeks during which

(4) that的用法

可以指人或物，指人时可以与who/whom交换。当你指着东西的时候，你可以和which交换。可以在文章中用作主语、宾语、标语等。制作介词宾语时，介词不能在that前提及。

E.g . the train ran over and killed a boy and his dog that were just crossing the track。/the picture(that)we are looking at was

(5) as的用法

多做主语、宾语、标语等的话，先行词经常出现在the same、such、as、so等修饰的定语从句或先行词在整个主句不受限制的定语从句中。As在句子中可以指人或事物，也可以指整个句子，这时子句可以位于句子的开头、句尾或句子中。

E.g . the explorer took only such men and things as he really needed into the jungle。/Shakespeare is a famous writer、Aseverybody knows.=aseverybody knows，Shakespeare is a famous写入程序。=Shakespeare、as everybody knows、is a famous写入程序

2.关系副词：在定语从句中用作副词，通常有where、when、why等。

(1) where的用法

代替表示地点的先行词，在从句中用作地点副词，经常可以被in which、on which、at which、to which等代替。

e . g . potato can be grown in places where/in which it is too cold to grow rice .

(2) when的用法

代替表示时间的先行词，在从句中使用时间副词，通常可以用on which、in which、at which、during which等代替。

E.g . I shall never forget the days when/in which we worked on the farm .

(3) why的用法

一般跟着先行词reason走，可以在句子中做思维副词，用for which代替，在引导非限制性定语从句时不能使用why，但可以使用for which。

e . g . can you tell me the reason why/for which the car broke down .

结论：定语从句关系词的选择关键是观察定语从句中缺少哪些句子成分。如果主语或宾语不足，则使用关系代词，然后根据先行词所指的人或事物等决定使用什么关系代词。如果副词不足，就使用关系副词。如果定语不够，就使用whose。另外，可能想把先行词放入定语节，这取决于从句中制作的成分。