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。在从句中所起作用如下：
(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 2,Whose 用来指人或物
(只用作定语, 若指物，它还可以同of which互换，指人的时候也可以用of whom 代替）
(1) They rushed over to help the man whose car had broken down. 那人车坏了，大家都跑过去帮忙。
(2) Please pass me the book whose color (the color of which) is green. 请递给我那本绿皮的书。
(3) The 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。 3,which, that
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） 4,as
as 可以做主语，也可以作宾语，作宾语时一般不省略，指人或者物。常与such as,the same as等短语连用。 5，as与which的区别
①as可以放在句首，而which不行 ②as表示主观，which表示客观事实 ③as引导非限制性定语从句 关系副词引导的定语从句 when指时间
在定语从句中做时间状语也可做连接词用
(1) I still remember the day when I first came to the school. (2) The time when we got together finally came. where指地点
在定语从句中做地点状语
(1) Shanghai is the city where I was born.
(2) The house where I lived ten years ago has been pulled down.
why指原因
在定语从句中做原因状语
(1) Please tell me the reason why you missed the plane. (2) I don’t know the reason why he looks unhappy today. 注意：关系副词引导的从句可以由“介词+关系代词”引导的从句替换
(1) The reason why/ for which he refused the invitation is not clear,
(2) From the year when/in which he was going to school he began to know what he wanted when he grew up. 先行词和关系词
1)Whoever spits in public will be punished here. (Whoever 可以用 anyone who 代替)
2)The parents will use what they have to send their son to technical school. (what 可以用all that代替)
但这两句句子已经不是定语从句了，是名词性从句。因为定语从句一定要有先行词，而名词性从句没有。将“Whoever”、“what”分别用“Anyone who”、“all that”代替后，才是定语从句，先行词分别是“Anyone”、“all”。
由as, which 引导的非限定性定语从句，as和which可代整个主句，相当于and this或and that。As一般放在句首，which在句中。
As we know, smoking is harmful to one's health. The sun heats the earth, which is very important to us. 典型例题
1）Alice received an invitation from her boss, ___came as a surprise.
A. it B. that C. which D. he 答案C。
此为非限定性从句，不能用 that修饰，而用which.，it 和he 都使后句成为句子，两个独立的句子不能单以逗号连接。况且选he句意不通。
2）The weather turned out to be very good, ___ was more than we could expect.
A. what B. which C. that D. it 答案B。
which可代替句子，用于非限定性定语从句，而what不可。That 不能用于非限定性定语从句，it不为连词，使由逗号连接的两个句子并在一起在英语语法上行不通。
3）It rained hard yesterday, ____ prevented me from going to the park..
A. that B. which C. as D. it 答案B。
as 和which在引导非限制性定语从句时，这两个关系代词都指
主句所表达的整个意思，且在定语从句中都可以作主语和宾语。但不同之处主要有两点：
(1） as 引导的定语从句可置于句首，而which不可。 (2） as 代表前面的整个主句并在从句中作主语时，从句中的谓语必须是系动词；若为行为动词，则从句中的关系代词只能用which.。
在本题中，prevent由于是行为动词，所以正确选项应为B。 As 的用法例
1. the same? as；such?as 中的as 是一种固定结构, 和??一样??。
I have got into the same trouble as he (has). 例2. as可引导非限制性从句，常带有'正如'。 As we know, smoking is harmful to one's health. As is known, smoking is harmful to one's health. As是关系代词。例1中的as作know的宾语；
例2中，它充当从句的主语，谓语动词know要用被动式。 一、“as / which” 特殊定语从句的先行成分
1. 形容词或形容词短语作先行成分, 具有形容词意义的介词短语也可以充当先行成分，如：
My grandmother’s house was always of great
importance to me, as my own is.
在一定的语言环境里，有些名词可以具有形容词的性质。这些名