功能：相当于形容词，修饰名词或代词，在句中作定语
位置：定语从句置于被修饰词之后
例如：He is the man who studies in USA.
本句中涉及到三个概念：
1.先行词即 the man：被定语从句饰的词称为先行词
先行词一般是名词和不定代词，如:some-, any-, every-和no与-boy, -thing的合成词；或all、none、any、some、that、those等代词。数词也可以作先行词，人称代词也同样可作先行词。
2. 引导词即who:起连接的作用
引导词分两类：关系代词和关系副词
关系代词：who;which;that;whose;whom
关系副词：when;where;why
3.从句即studies in USA.：引导词之后的句子。
1. 关系代词 : who;which; that; whom; whose
其中关系代词who;which;that;whom用于后面的从句缺主语或宾语的情况
(1) who:指代先行词是人(可做从句的主语或宾语)
She is the girl who I teach in China.(从句缺宾语)
That is the man who teaches us English.(从句缺主语)
(2) which:指代先行词是物（做从句的主语或宾语）
That is the book which I want.(从句缺宾语)
There is a bird which stands in the building.(从句缺主语)
(3) that: 指代先行词是人或物（做从句的主语或宾语）
She is the girl who/that I teach in China.(从句缺宾语)
There is a bird which/that stands in the building.(从句缺主语)
(4) whom;指代先行词是人，但只用于从句是缺宾语的情况。
She is the girl who/whom I teach in China.(从句缺宾语)
That is the man whom teaches us English.(从句缺主语,本句用whom是错误的)
(5) whose:指代从句中缺定语的情况（表示所属关系）
That is the building whose windows broke last night.(本句中的windows是属于the building构成一种所属关系即缺少building’s)
2.关系副词：when;where;why(该三个词主要用于从句是不缺主语或是宾语的情况即从句是个完整句)
(1) when:指代先行词表时间如：time；month;year;occasion等(形式上等于介词 which)
I still remember the day when /on which I came to Beijing.(该从句是一个完整的句子，先行词是the day在从句中需要加个介词on来做状语的成分即I came to Beijing on the day)
(2)where :指代先行词是地点的词如：place;house;school;也可以是模糊的词如：point;stage;work;situation等(形式上等于介词 which)
That is the place where/in which I was born .(该从句I was born也是一个完整句，即不缺主语或宾语的，此时where可以换成in which)
(3)why:指代先行词是表原因的词即：reason（形式上只能是等于for which)
That is the reason why/for which he was late for school.
三.非限制性定语从句
非限制性定语从句起补充说明作用，缺少也不会影响全句的理解。在非限制性定语从句的前面往往有逗号隔开。
The house, which I bought last year, has a lovely garden.
He seems not to have grasped what I meant, which greatly upsets me.
I still remember the day, when I came to Beijing.
四.总结
定语从句三步法：
1. 找出先行词
2. 判断从句是不是完整句:
完整句考虑关系副词：when;where; why
不是完整句考虑关系代词：that;who;which;whom;whose
3. 选择合适的关系词
注意：
（1）定语从句中是没有引导词what;
（2）非限制性定语从句中无引导词that；
The house, that I bought last year, has a lovely garden.(错误)
改为The house, which I bought last year, has a lovely garden.
（3）先行词做从句的宾语时，可省引导词
That is the book which I want.(从句缺宾语)可改为That is the book I want.
（4）在定语从句中，当先行词指物时，下列情况的关系词宜用that而不用which
1．先行词被①形容词最高级 ②序数词 ③数词几种词修饰或被 ④only、any、few、little、no、all、one of等修饰时。
2．先行词为all、much、little、none、few、one、something、anything等不定代词时。
3．先行词中既有人又有物时。
He was looking pleasantly at te children and parcels that filled his bus.
4．先行词在主句中作表语关系词在从句中作表语时。
The village is no longer the one that was 5 years ago.
5．当主句中含有疑问词which时。
Which are the books that you bought for me ?
（5）宜用which而不用that的情况
①在非限制性定语从句中
②在关系词前有介词时
③当先行词本身是that时
④当关系词离先行词较远时
