1、在後置的非限制性定語從句中代替上文出現的事物或情況（單數或復數）。
2、在問句中作為疑問代詞，在名詞從句中作為連接代詞用，其含義為“哪個、哪些”。
3、如果非限制性定語從句的先行詞不是指主句內容，而是指具體的人、時間、地點，則不用which。
[解題過程]
這裏主要講解壹下在定語從句中的用法：
壹、關系代詞that和which在定語從句中作主語和賓語，放在先行詞和定語從句之間，起聯系作用。指物時，壹般情況下可互換。如：
Here is the pen that/which you lost yesterday.(that/which指物作賓語)
A plane is a machine that/which can fly.(that/which指物，作主語)
He is the most careful boy that I know.(that指人,作賓語)
The man that works in this room is a chemist.(that指人,作主語)
二、that,which在從句中作主語時，不能省略。作賓語時在口語中和非正式問題中經常省略。如上例1和中的that,which都可省略。
三、 關系代詞在主語從句中作主語時，從句的謂語動詞的人稱和數須和先行詞壹致。
Wheat is a plant that/which is grown in the north of China.
四、在定語從句中，關系代詞that和which指物時，壹般沒有區別，但下列幾種情況中不能互換：
1 只能用that的情況：
a)先行詞為all,everything,nothing,anything,little等不定代詞時（something後也可用which）：
Is there anything (that) I can do for you in town?
That’s all (that) I know.
b)先行詞被序數詞所修飾時：
The first thing (that) we should do is to work out a plan.
c) 先行詞被形容詞最高級所修飾時：
This is one of the most exciting football games (that) I have ever seen.
The nanjing yangtse river bridge is the longest bridge (that) they have ever seen.
d)先行詞 被only,very,no,any,same,little,one of等詞所修飾時：
This is the only thing (that) we can do now.
He gave me the same pen (that) he borrowed from me.
Do you have any books that were written by Lu Xun?
This is the very book (that) I want to read.
e)當先行詞中既有人又有物時：
The man and the car (that) you want to see are all here.妳相見的人和車都在這裏。
f) that可指人，which則不能：
Who is the man that is reading the newspaper over there?
g) 在“the time when”等結構和某些時間狀語中，when常被that代替，而且that往往省略：
I may leave here any time I want to.
During the time I was there I visited him twice.
h)way後面可根由that引導的定語從句，但that壹般都省略：
That was the way she looked after us.
2 在下列情況中，只能用which:
a) 放在介詞後面做介詞賓語時：
This is the question about which we’ve had so much discussion.
但是，如把介詞放在從句的後部，這時which就可換為that並常省略。如上例可改為：
This is the question (that) we’ve had so much discussion about.
b) 在非限制性定語從句，無論指物指人，都不能用that。指物時，只能用which和其他關系代詞。如：
All the books there,which have beautiful pictures in them,were written by him.