要学定语从句，得知道什么是定语。定语是用来限定、修饰名词或代词的。例如，a good book, good就是定语。那么复合句中修饰名词或代词的从句就叫做定语从句。定语从句由关系代词或关系副词引导，相当于形容词，在句中作定语。被定语从句修饰的词叫先行词，定语从句一般放在先行词的后面。引导定语从句的关联词称为关系词，关系词有关系代词和关系副词。关系代词有that, which, who, whom, whose等，绝对没有what;关系副词有where, when, why,how等。关系词放在先行词和定语从句之间，起连接作用，同时又作定语从句的某个成分。
一、关系代词引导的定语从句
1、由who, whom, that，Whose引导的从句
这些词代替的先行词是人的名词或代词，who作主语指人，whom作宾语指人，that既可作主语又可作宾语(作宾语可以省略)，可以指人也可以指物。Whose 用来指人或物，只用作定语。
(1) Is he the man who/that wants to see you? 他是那个想见你的男人吗?(who/that在从句中作主语)
(2) He is the man whom/ that I saw yesterday. 他就是我昨天见的那个人。(whom/that在从句中作宾语)
(3) The man whom you spoke to just now is our English teacher 你刚刚说话的那个男人是我们的英语老师
(4) he man whose son is a doctor is our professor. 那个儿子是医生的男人是我们的教授。
2、由which, that引导的从句
它们所代替的先行词是事物的名词或代词，在从句中可作主语、宾语等，作宾语时可以省略，例如：
(1) Prosperity which / that had never been seen before appears in the countryside. 农村出现了前所未有的繁荣。(which / that在从句中作主语)
(2) The package (which / that) you are carrying is about to come unwrapped. 你拿那个包裹快要散开了。(which / that在从句中作宾语)
注意： 代表物时多用which，但在下列情况中用that而不用which：
a)先行词是anything, everything, nothing , none等不定代词时;
b)先行词由every, any, all, some, no, little, few, much等修饰时，这时的that常被省略;
c)先行词前有序数词或被形容词最高级修饰时;
d)先行词中既有人又有物时;
e)整个句中前面已有which，who,that时;
f)当先行词为物并作表语时;
g)先行词为one时;
h)先行词同时又被the only,the very,the same修饰时;
二、关系副词引导的定语从句
1、when指时间在定语从句中做时间状语也可做连接词用 I still remember the day when I first came to the school.
2、where指地点在定语从句中做地点状语 Shanghai is the city where I was born.
3、why指原因在定语从句中做原因状语 ，用在reason 后面。 Please tell me the reason why you missed the plane.
常见考法
对于定语从句的考查，多以单选的形式考查学生灵活运用的能力。一般情况下，常从关系词的意义和功能的角度考查，重点是that,which,who引导的定语从句。
典型例题：Youre the only person ______Ive ever met ______could do it.
A. who B. whom C. whom D. who
解析：先行词person后有两个定语从句,第一个从句省略了关系代词whom.因为, whom作从句中met的宾语,可以省略.第二个从句who could do it.who在从句中作主语,不可省略.
答案：D
误区提醒
当表示时间或地点的名词作先行词时，要判断出它们在从句中作状语还是主语或宾语。作状语时用关系副词，反之用关系代词。
典型例题：I can never forget the day _______ we worked together and the day ______ we spent together.
A. when; which B. which; when C. what; that D. on which; when
解析：. 两个先行词the day都是表示时间的名词,但第一个空白处要填的关系词在从句中作状语,因此要用关系副词when. 第二个空白处要填的关系词在从句中作动词spent的宾语,因此要用关系代词which或that来引导定语从句.
答案：A