关系代词有：who, whom, whose, that, which, as。
关系副词有：when, where, why, how。
1) who, whom, that 这些词代替的先行词是人的名词或代词，在从句中所起作用如下：
Is he the man who/that wants to see you?（who/that在从句中作主语）
He is the man whom/ that I saw yesterday.（whom/that在从句中作宾语）
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），例如：
Please pass me the book whose (of which) cover is green.
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，例如：A prosperity which / that had never been seen before appears in the countryside.（which / that在句中作宾语）
The package (which / that) you are carrying is about to come unwrapped. (which / that在句中作宾语)
关系代词that和which 都可以指物，that 和Who 都可以指人，其用法区别：
(错)The tree, that is four hundred years old, is very famous here.
We depend on the land from which we get our food.
c) 多用who 的情况
A friend who helps you in time of need is a real friend.
②先行词为those, people 时
Those who were either fools or unfit for their offices could not see the cloth.
③先行词为all, anyone, ones, one 指人时
One who doesn't work hard will never succeed in his work.
There is a stranger who wants to see you.
A new teacher will come tomorrow who will teach you German.
The student who was praised at the meeting is the monitor that is very modest and studies very hard.
There is a teacher who is always ready to help others and who enjoys what he does.
a)在不定代词，如：anything, nothing, everything, all, much, few, any, little等作先行词时，只用that，不用which。
All that is needed is a supply of oil.
Finally, the thief handed everything that he had stolen to the police.
b)先行词有the only, the very, the just修饰时，只用that。
He is the very man that helped the girl out of the water.
The first English book that I read was "The Prince and the Pauper" by Mark Twin.
He talked about the teachers and schools that he visited.
e)当主句是以who 或which 开始的特殊疑问句时，用that 以避免重复。
Who is the person that is standing at the gate.
He is not the man that he used to be.
关系副词when, where, why, how的含义相当于"介词+ which"结构，因此常常和"介词+ which"结构交替使用，例如：
There are occasions when (on which) one must yield.
Beijing is the place where (in which) I was born.
Is this the reason why (for which) he refused our offer?
I'm surprised the way how (by which) he works out the problem.
如：They set up a state for their own , where they would be free to keep Negroes as slaves.
Is this the book which (that) she was looking for?
3、名词/数词/代词 /形容词最高级 + 介词 + 关系代词引导定语从句
She has written a book , the name of which I have forgotten.
There are fifty-five students in our class , all of whom are working hard.
There are five continents in the world , the largest of which is Asia.
4、as, which 引导非限定性定语从句的差别
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.
As is know, smoking is harmful to one's health.
(1) as 引导的定语从句可置于句首，而which不可。
As we all know, he never smokes.
(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。
(3)非限定性定语从句中出现expect, think, suppose 等表示猜测、想象、预料等时。
She succeeded in her doing the research work , as we expected.
(4)As 的用法 the same… as; such…as 中的as 是一种固定结构, 和……一样……。
(5)I should like to use the same tool as is used here.
We should have such a dictionary as he is using.