一、概念
在複合句中，修飾名詞或代詞的從句叫定語從句。引導定語從句的詞叫關係詞。被修飾的名詞或代詞叫先行詞。定語從句一般放在先行詞的後面。
二、 引導詞
﹙1﹚關係代詞：that/who/whom/which/as
﹙2﹚關係副詞：when/where/why
三、分類
根據定語從句與先行詞的關係，定語從句可分為限制性定語從句及非限制性定語從句。
1、限制性定語從句緊跟先行詞，主句與從句不用逗號分開，從句不可省去。例如：
Any man that / who has a sense of duty won’t do such a thing.
任何有責任感的人都不會做這樣的事。
2、非限制性定語從句主句與從句之間有逗號分開，起補充說明作用，如省去，意思仍完整。例如：
The house,which we bought last month,is very nice.
這幢房子很漂亮,是我們上個月買的.
注意：非限定性定語從句中不能使用關係代詞that。
四、關係代詞的用法
1、that: 可指人或物;在定語從句中作主語，賓語，表語。(指人時，相當於who或 whom;指物時，相當於which)(一般不用於非限制性定語從句; 不可置於介詞後作賓語) 如：
（1） A letter that/which is written in pencil is difficult to read. (主語)
（2）Do you know the gentleman that/who spoke just now?
（3）You can take anything ( that) you like. (賓語)
（4）What is the question (that/which) they are talking about?
（5）Here is the man ( who/whom/that ) you want to see.
（6）She's no longer the girl ( that) she used to be before.(表語)
（6）Our hometown is no longer the one (that) it used to be.= Our hometown is not the same as it used to be.= Our hometown is different from what it used to be.= Our hometown is not what it used to be.
2、which: 指物;在定語從句中作主語，賓語，表語，定語。如：
（1） The book which/that was on the desk was bought by my father.(主語)
（2）2. The book (which/that) I bought yesterday is very interesting.(賓語)
（3）The factory in which his father works is far from here.
（4）He was proud, which his brother never was. (表語)
（5）Tom spent four years in college, during which time he learned French.(定語)
（6） He may be late, in which case we ought to wait for him.
3、who, whom, whose:
who: 主格, 在從句中作主語，在口語或非正式用法中作賓語; 只可指人
whom: 賓格，在從句中作賓語; 只可指人
whose: 屬格，在從句中作定語，可指人也可指物。
（1）I like the students who/that work hard. (主語)
（2）All who heard the story were amazed. (代詞如he, they, any, those, all, one等後多用who.)
（3）Chaplin, for whom life had once been very hard, was a success as an actor. (賓語)
（4）He's a man from whom we should learn. = He's a man (whom/who/that) we should learn from.
比較：He is the student whom you think to be worthy of your praise.
He is the student who you think is worth praising.
（5） A child whose parents are dead is called an orphan.(指人)
I'd like a room whose window faces south. (指物)=I'd like a room of which the window faces south. =I'd like a room the window of which faces south.
There is a teapot shaped like a Chinese duck, out of whose mouth tea is supposed to come
關係代詞作介詞賓語:
關係代詞在定語從句中用作介詞賓語時, 介詞可放於從句之首, 也可放於從句之末. 但以放於句首較為正式.(介詞前置，必須注意不影響動詞詞組的含義。關係代詞who 和 that 用作介詞賓語時, 介詞必須放在句末.)如:
（1） This is the book for which you asked.
=This is the book (that/which) you asked for.
（2）Do you know the person with whom I shook hands?= Do you know the person (whom/who/that) I shook hands with?
（3）The beginning of agriculture was a big step in human progress with which nothing could compare until our information age.
（4）In the dark street, there wasn't a single person to whom she could turn for help.
Is this the factory to which you paid a visit last week?
Is this factory the one to which you paid a visit last week?
（5）This is the girl whom they are looking after. (介詞after與look構成固定詞組，不可前置。再如：look at, look for, look after, take care of等)
4、as 的用法:(as 引導定語從句, 在定語從句中作主語、賓語、表語)
(1) 如為限制性的，多用於the same …as ; the same as;such …as …; as many/much as;so …as等結構中。如：
I have the same book as you (have). 我有一本和你的一樣的書。
Her attitude to him was quite the same as it had always been. (關係代詞as和指示代詞same連用, 在從句中用作表語, 先行詞是same.)
Why didn't you mention that in face of the police just now?
I thought it was such a minor detail as was hardly worth mentioning.
We will only discuss such problems as have something to do with our own interests.
Don't do such things as you are not sure about.
There is no such place as you dream of in all this world.
比較：I live in the same house that he used to live in.
I'm wearing the same shirt as you wore yesterday.
比較：Here is so big a stone as no one can lift. (定語從句)
Here is so big a stone that no one can lift it.(結果狀語從句)
(2) 如為非限制性的，多單獨引導一個定語從句，這種定語從句可置於句首，句中或句尾，譯為"正如，這一點"。(動詞常為know, see, expect, point out, etc.)
As we all know, smoking is harmful to one's health . (as 作賓語)
=As is known to all, smoking is harmful to one's health . (as 作主語)=It's known to all that smoking is harmful to one's health.或:Smoking is harmful to one's health , as we all know .(as 作賓語)或: Smoking, as we all know, is harmful to one' health.
He was a foreigner, as I knew from his accent. (賓語, 先行詞是前面整個句子)
5、but的用法:
but用作關係代詞, 其意思相當於who/that…not…, "沒有……不……", 如
（1）There is not one of us but wishes to help you.
（2）There is no tree but bears some fruit.
（3）There are very few but admire his talents.
五、關係副詞的用法
1、when指時間，其先行詞表示時間，在句中作時間狀語。
例如：This was the time when he arrived.
這是他到達的時間。(when=at which)
2、where指地點，其先行詞表示地點，在句中作地點狀語。
例如：This is the place where he works.
這是他工作的地點。(where=at /in which)
3、 why 指原因，其先行詞是原因，起原因狀語作用。
例如：Nobody knows the reason why he is often late for school.
沒人知道他為什麼上學總遲到。(why=for which)
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