定語從句（Attributive Clauses）在句中做定語，修飾壹個名詞或代詞，被修飾的名詞,詞組或代詞即先行詞。定語從句通常出現在先行詞之後，由關系詞（關系代詞或關系副詞）引出。
關系代詞有：who, whom, whose, that, which等。
關系副詞有： when, where, why等。
18.1 關系代詞引導的定語從句
關系代詞所代替的先行詞是人或物的名詞或代詞，並在句中充當主語、賓語、定語等成分。關系代詞在定語從句中作主語時，從句謂語動詞的人稱和數要和先行詞保持壹致。
1）who, whom, that
這些詞代替的先行詞是人的名詞或代詞，在從句中所起作用如下：
Is he the man who/that wants to see you?
他就是妳想見的人嗎？（who/that在從句中作主語）
He is the man whom/ that I saw yesterday.
他就是我昨天見的那個人。（whom/that在從句中作賓語）
2) Whose 用來指人或物，(只用作定語, 若指物，它還可以同of which互換）, 例如：
They rushed over to help the man whose car had broken down. 那人車壞了，大家都跑過去幫忙。
Please pass me the book whose (of which) cover is green. 請遞給我那本綠皮的書。
3）which, that
它們所代替的先行詞是事物的名詞或代詞，在從句中可作主語、賓語等，例如：
A prosperity which / that had never been seen before appears in the countryside. 農村出現了前所未有的繁榮。（which / that在句中作賓語）
The package (which / that) you are carrying is about to come unwrapped. 妳拿的包快散了。(which / that在句中作賓語）
關系副詞引導的定語從句
關系副詞可代替的先行詞是時間、地點或理由的名詞，在從句中作狀語。
1）when, where, why
關系副詞when, where, why的含義相當於"介詞+ which"結構，因此常常和"介詞+ which"結構交替使用，例如：
There are occasions when (on which) one must yield. 任何人都有不得不屈服的時候。
Beijing is the place where (in which) I was born. 北京是我的出生地。
Is this the reason why (for which) he refused our offer? 這就是他拒絕我們幫助他的理由嗎？
2）that代替關系副詞
that可以用於表示時間、地點、方式、理由的名詞後取代when, where, why和"介詞+ which"引導的定語從句，在口語中that常被省略，例如：
His father died the year (that / when / in which) he was born. 他父親在他出生那年逝世了。
He is unlikely to find the place (that / where / in which) he lived forty years ago. 他不大可能找到他四十年前居住過的地方。
判斷關系代詞與關系副詞
方法壹： 用關系代詞，還是關系副詞完全取決於從句中的謂語動詞。及物動詞後面無賓語，就必須要求用關系代詞；而不及物動詞則要求用關系副詞。例如：
This is the mountain village where I stayed last year.
I'll never forget the days when I worked together with you.
判斷改錯（註：先顯示題，再顯示答案，橫線；用不同的顏色表示出。）
（錯） This is the mountain village where I visited last year.
（錯） I will never forget the days when I spent in the countryside.
（對） This is the mountain village (which) I visited last year.
（對） I'll never forget the days (which) I spent in the countryside.
習慣上總把表地點或時間的名詞與關系副詞 where, when聯系在壹起。此兩題錯在關系詞的誤用上。
方法二： 準確判斷先行詞在定語從句中的成分(主、謂、賓、定、狀)，也能正確選擇出關系代詞/關系副詞。
例1. Is this museum ___ you visited a few days age?
A. where B. that C. on which D. the one
例2. Is this the museum ____ the exhibition was held.
A. where B. that C. on which D. the one
答案：例1 D，例2 A
例1變為肯定句： This museum is ___ you visited a few days ago.
例2變為肯定句： This is the museum ___ the exhibition was held.
在句1中，所缺部分為賓語，而where, that, on which都不能起到賓語的作用，只有the one既做了主句的表語，又可做從句的賓語，可以省略關系代詞，所以應選D。
而句2中, 主、謂、賓俱全，從句部分為句子的狀語表地點，既可用副詞where，又因 in the museum 詞組，可用介詞in + which 引導地點狀語。而此題中，介詞on 用的不對，所以選A。
關系詞的選擇依據在從句中所做的成分，先行詞在從句中做主、定、賓語時，選擇關系代詞 (who, whom, that, which, whose)； 先行詞在從句中做狀語時，應選擇關系副詞 ( where 地點狀語，when 時間狀語，why 原因狀語) 。
限制性和非限制性定語從句
1) 定語從句有限制性和非限制性兩種。限制性定語從句是先行詞不可缺少的部分，去掉它主句意思往往不明確；非限制性定語從句是先行詞的附加說明，去掉了也不會影響主句的意思，它與主句之間通常用逗號分開，例如：
This is the house which we bought last month. 這是我們上個月買的那幢房子。（限制性）
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。（非限制性）
2) 當先行詞是專有名詞或物主代詞和指示代詞所修飾時，其後的定語從句通常是非限制性的，例如：
Charles Smith, who was my former teacher, retired last year. 查理·史密斯去年退休了，他曾經是我的老師。
My house, which I bought last year, has got a lovely garden. 我去年買的的那幢房子帶著個漂亮的花園。
This novel, which I have read three times, is very touching. 這本小說很動人，我已經讀了三遍。
3) 非限制性定語從句還能將整個主句作為先行詞, 對其進行修飾, 這時從句謂語動詞要用第三人稱單數，例如：
He seems not to have grasped what I meant, which greatly upsets me. 他似乎沒抓住我的意思，這使我心煩。
Liquid water changes to vapor, which is called evaporation. 液態水變為蒸汽，這就叫做蒸發。
說明：關系代詞that和關系副詞why不能引導非限制性定語從句。
介詞+關系詞
1）介詞後面的關系詞不能省略。
2）that前不能有介詞。
3） 某些在從句中充當時間，地點或原因狀語的"介詞+關系詞"結構可以同關系副詞when 和where 互換。
This is the house in which I lived two years ago.
This is the house where I lived two years ago.
Do you remember the day on which you joined our club?
Do you remember the day when you joined our club?
as, which 非限定性定語從句
由as, which 引導的非限定性定語從句，as和which可代整個主句，相當於and this或and that。As壹般放在句首，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.
典型例題
1）Alice received an invitation from her boss, ___came as a surprise.
A. it B. that C. which D. he
答案C. 此為非限定性從句，不能用 that修飾，而用which.，it 和he 都使後句成為句子，兩個獨立的句子不能單以逗號連接。況且選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，用於非限定性定語從句，而what不可。That 不能用於非限定性定語從句，it不為連詞，使由逗號連接的兩個句子並在壹起在英語語法上行不通。
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在引導非限制性定語從句時，這兩個關系代詞都指主句所表達的整個意思，且在定語從句中都可以作主語和賓語。但不同之處主要有兩點：
(1） as 引導的定語從句可置於句首，而 which不可。
(2） as 代表前面的整個主句並在從句中作主語時，從句中的謂語必須是系動詞；若為行為動詞，則從句中的關系代詞只能用which.。
在本題中，prevent由於是行為動詞，所以正確選項應為B。
As 的用法
例1. the same… as；such…as 中的as 是壹種固定結構, 和……壹樣……。
I have got into the same trouble as he (has).
例2. as可引導非限制性從句，常帶有'正如'。
As we know, smoking is harmful to one's health.
As is know, smoking is harmful to one's health.
As是關系代詞。例1中的as作know的賓語；例2中，它充當從句的主語，謂語動詞know要用被動式。
先行詞和關系詞二合壹
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)
what/whatever;that/what; who/whoever
1）what = the thing which；whatever = anything
What you want has been sent here.
Whatever you want makes no difference to me.
2) who= the person that whoever= anyone who
（錯）Who breaks the law will be punished.
（錯）Whoever robbed the bank is not clear.
（對）Whoever breaks the law will be punished.
（對）Who robbed the bank is not clear.
3) that 和 what
當that引導定語從句時 ，通常用作關系代詞，而引導名詞性從句時，是個不充當任何成分的連接詞。賓語從句和表語從句中的that常可省略。What只能引導名詞性從 句， 用作連接代詞，作從句的具體成分，且不能省略。
I think (that) you will like the stamps.
What we need is more practice.
關系代詞that 的用法
1）不用that的情況
a) 在引導非限定性定語從句時。
(錯) The tree, that is four hundred years old, is very famous here.
b) 介詞後不能用。
We depend on the land from which we get our food.
We depend on the land that/which we get our food from.
2) 只能用that作為定語從句的關系代詞的情況
a) 在there be 句型中，只用that，不用which。
b) 在不定代詞，如：anything, nothing, the one, all, much, few, any, little等作先行詞時，只用that，不用which。
c) 先行詞有the only, the very修飾時，只用that。
d) 先行詞為序數詞、數詞、形容詞最高級時，只用that。.
e) 先行詞既有人，又有物時。
舉例：
All that is needed is a supply of oil.
所需的只是供油問題。
Finally, the thief handed everything that he had stolen to the police.
那賊最終把偷的全部東西交給了警察。