定义：定语从句是在句子中担任定语功能修饰主句中的某一名词或代词的句子。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。关系词连接先行词与定语从句，并代替先行词在从句中作一定的句子成分，有关系代词和关系副词两种。
1.关系代词：在定语从句中作主语、宾语、表语、定语，常用的有who, whom, whose, that, which, as 等。
(1) who/whom 的用法
两者都可指人，但who在定语从句中作主语、宾语，而whom只作宾语。
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：whom不位于介词后时可用who代替，也可省略，而在非限制性定语从句中，whom不可用who代替。
e.g. Do you know the girl (who/whom) they are talking about?
(2)whose 的用法
既可指人，也可指物，在定语从句中作定语。
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"常可用“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) which的用法
一般指物，在定语从句中作主语、宾语，偶尔作定语。
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 time they drank all the wine I had.
(4) that 的用法
可指人或物，指人时可与who/whom互换；指物时可与which互换。在从句中可作主语、宾语、表语等。作介词宾语时，介词不可提到that之前。
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 drawn by a fifteen-year-old student.
(5) as 的用法
多作主语、宾语、表语等，常出现于先行词被the same, such, as, so等修饰的定语从句或先行词是整个主句的非限制定语从句中。as在从句中可指人或物，也可指整个句子，此时从句可位于句首、句尾或句中。
e.g. The explorer took only such men and things as he really needed into the jungle. / Shakespeare is a famous writer, as everybody knows. = As everybody knows, Shakespeare is a famous writer. = Shakespeare, as everybody knows, is a famous writer.
2. 关系副词：在定语从句中作状语，常用的有where, when, why等。
(1) where的用法
代替表示地点的先行词，在从句中作地点状语，常可用in which, on which, at which, to which等取代。
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，在从句中作时间状语，常可用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
一般跟在先行词reason后，在从句中作原因状语，可用for which代替，引导非限制性定语从句时不能用why，但可用for which。
e.g. Can you tell me the reason why/for which the car broke down.
结语：定语从句关系词的选择的关键在于观察定语从句中缺少何种句子成分。若缺少主语或宾语，则用关系代词，然后再依据先行词所指的是人或物等确定用哪个关系代词；若缺少状语，则用关系副词；若缺少定语则用whose。另外也可把先行词试着放进定语从句，根据其在从句中所作的成分而定。