在複合句中作定語，修飾主句中某一名詞或代詞的從句，叫做定語從句。從句在複合句子中的作用相當於形容詞，故又稱為形容詞性從句。
一. 先行詞
被定語從句修飾的名詞、代詞稱為先行詞。先行詞一般出現在定語從句之前。
Do you know the girl who is singing on the stage?
你認識在舞臺上唱歌的那個姑娘嗎?( the girl為先行詞)
This is the museum which was built last year.
這就是去年建成的博物館。( the museum為先行詞)
I’ll forever cherish the days when I studied abroad.
我將永遠懷念我在國外求學的日子。( the days為先行詞)
Let’s find a place where we can have a picnic.
我們找一個能野餐的地方吧。( a place為先行詞)
二. 關係詞
引導定語從句的詞被稱為關係詞。關係詞根據其在定語從句中的功能可分為關係代詞和關係副詞。關係代詞在從句中作主語、賓語、表語、定語等，關係副詞在從句中作狀語。
Speaking of the USA, New York is the first thing that comes to mind.
說到美國，人們最先想到的就是組約。(作主語)
I’ll cherish the moment when I met Lisa.
我將懷念我和麗莎見面的那一時刻。(作狀語)
1. 關係代詞
常見的關係代詞如下表
（1）who, whom, that
He is the man who/that lives next door.
他就是住在隔壁的那名男子。 (who/that在定語從句中作主語，指代the man)
Mr White invited many friends to his party (who/whom/that) he respected.
懷特先生邀請了很多他尊敬的朋友參加他的聚會。(who/whom/that在定語從句中作respected的賓語，指代 many friends，可省略)
注意：
1）以下兩種情況在定語從句中作賓語只能用whom，且不可省略：介詞後賓語的關係代詞只能用whom；在非限制性定語從句中，只能用whom作賓語。
I have many friends to whom I am going to send postcards.
我有很多要給他們寄賀卡的朋友。（whom在定語從句中作介詞to後面的賓語）
A young man had a new girl friend, whom he wanted to impress.
一個年輕的小夥子新交了一個女朋友,他想給她留下深刻印象.（非限制性定語從句中，作賓語用whom）
2）宜用who不宜用that的情況
① 先行詞是指人的不定代詞時，如果先行詞是one, ones, anyone時。
The ones who flatter me don't please me,
那些奉承我的人並不能取悅我
We don't want anyone who can't afford the tuition fees to drop out of school.
我們不想讓任何一個交不起學費的人輟學。
②先行詞為those時。
Those who have good manners will be highly respected.
那些有禮貌的人會備受尊敬。
③當先行詞指人並含有較長的後置定語或在被分割的定語從句中時。
Professor Smith is coming soon who will give us a talk on how to learn English.
史密斯教授不久就要來給我們作一場關於如何學英語的講座。
The boy was crying bitterly who lost his way while looking for his mother.
這個男孩在找媽媽的過程中迷了路，哭得很傷心。
④在 there be開頭的句子中。
There is an old man who wants to see you.
有位老人要見你。
There are many young men who are against him.
有許多年輕人反對他。
（2） which, that
The river which/that runs through the centre of the city brings us lots of pleasure.
穿過市中心的那條河給我們帶來很多歡樂。（which/that在定語從句中作主語）
This is the book (which/that) you are looking for.
這正是你在找的書。（which/that在定語從句中作賓語，可省略）
注意：
1) 指物時只能用which不能用that的兩種情況，且不能省略。
① 在定語從句中作介詞的賓語。
The house in which I used to live has become a shoeshop.
我過去住的房子現在變成一家鞋店了。
The pen with which you are writing is Jefr's.
你現在正用來寫字的那支鋼筆是傑夫的。
② 在非限制性定語從句中。
These apple trees，which I planted three years ago，have not borne any fruit.
這些蘋果樹是我三年前栽的，還沒有結過果實。
2）在下列情況中，關係詞指物時，只能用that引導，不能用 which引導。
①當先行詞是all, little, much, none, everything, anything, nothing等不定代詞時。
Is there anything that you don't understand?
你有什麼不懂的地方嗎?
Tom told his mother all that had happened.
湯卻把發生的所有事情都告訴了他的媽媽。
②當先行詞前面有 the only, the very, the last等修飾時。
This is the only book that I can find.
這是我能找到的唯一一本書。
③當序數詞或形容詞最高級修怖先行詞時。
This is the best book that I have ever read.
這是我讀過的最好的書。
④當先行詞前面有only, all, any, no等修飾時。
I want to read all the books that were written by Lu Xun.
我想讀所有魯迅寫的書。
⑤當主句是以疑問詞 which開頭的特珠疑問句時。
Which is the hotel that you like best?
哪個是你最喜歡的旅店?
⑥ 當先行詞既有人也有物時。
They talked of things and persons that they remembered.
他們談論他們記得的事和人。
3. whose
whose一般指人，但有時也指物，whose指代物時，相當於of which。在定語從句中作定語。whose指人時與of whom近似，但whose表示所有關係，而of whom則表示範圍（主要用於非限制性定語從句。
The building whose roof（=the roof of which）we can see from here is a hotel.
我們dao從這兒能看見房頂的那座建築是一家旅館。
I’d like a room whose window (=the window of which) overlooks the sea.
我想要一個透過窗戶能看到大海的房間。
Do you know the girl whose Japanese is excellent?
你認識那個日語非常優秀的姑娘嗎?
He is the man whose car was stolen last night.
他就是昨晚車被偷的那個人。
I met the newcomers, several of whom were Chinese.
我見到了那些新來的人，他們中有幾個是中國人。
注意：
關係代詞的省略。一般情況下that， which，whom作賓語時可以省略，但當關系代詞 which，whom在從句中作介詞的賓語且位於介詞後面時，不能省略。
I'll never forget the day on which I joined the club.
我永遠不會忘記我加入俱樂部的那一天。
2. 關係副詞
常見的關係副詞如下表：
（1）when
when在定語從句中作時間狀語。可由at/on/in/during which替代。
April Fool's Day is the day when (= on which ) people make fun ofothers.
愚人節是人們愚弄他人的日子。
Do you know the date when (= on which ) Lincoln was born?
你知道林肯的出生日期嗎?
（2）where
where在定語從句中作地點狀語，可由at/on/in/from/to which替代。
These are the few points where（= at which）we disagree with each other.
這些就是我們之間有分歧的幾點。
This is the mountain village where (=in which) I stayed Iast year.
這就是我去年待過的山村。
He did all his research in the room where (=in which) he lived.
他在自己住的房間裡做了所有的研究。
（3）why
why在定語從句中作原因狀語，可以用 for which代替。why不可引導非限制性定語從句，此時可用for which。
I have come around to explain the reason why（=for which）I was absent from the meeting.
我來是為了解釋我當時沒有到會的原因。
Is this the reason why（=for which） she refused our offer?
這就是她拒絕我們幫助她的理由嗎?
知識延伸
the reason why...與 the reason that
1)下面的句子是that引導的同位語從向，that不作成分，that後面的內容是解釋說明 reason的內容的，而why引導的定語從向並不解釋說明 reason的內容。
I don’t want to hear any reason why you were absent.
我不想聽你缺席的任何理由。（why引導定語從句，在從句中作原因狀語）
The reason that the car broke down dosen’t hold water.
車子壞了這一原因是站不住腳的。（that引導同位語從句，解釋reason的內容，在同位語從句中不作成分）
2）在下面這個句子中，不能用why，只能用which或that。
I don’t want to hear any reason (that/which) you might give.
我不想聽你可能給出的任何理由。(在這個定語從句中，that/ which作定語從句中謂語動詞give的賓語)
注意：以下的關係副詞可省略
1 ) the time, every time, each time, the moment後的關係副詞可省略。
By the time (when) he was fourteen years old, Einstein had learnt advanced mathematics all by himself.
到十四歲時，愛因斯坦已經自學了高等數學。
2) 在某些表示地點的名詞後，關係副詞有時也可以省略，如 the place等。
This is the place ( where) we met years ago.
這就是我們多年前相遇的地方。
3) 先行詞the reason後面的關係副詞可省略。
The reason he did that is quite clear.
他那樣做的理由是非常清楚的。
三. 定語從句中需注意的事項
- 主謂一致問題
關係代詞在定語從句中作主語時，定語從句中謂語動詞的單複數形式與先行詞應一致。
（1）先行詞是單數，從句的謂語動詞用單數; 先行詞是複數，從句的謂語動詞用複數。
Mr Smith, who is now downstairs, is asking to see you
史密斯先生要見你，他現在在樓下。
The Smiths, who are now downstairs, are asking to see you.
史密斯夫婦要見你，他們在樓下。
（2） “one of+複數名詞”後的定語從句的謂語動詞用複數形式;＂the (only) one of+複數名詞” 或“the very/ right one of+複數名詞”後的定語從句的謂語動詞用單數形式。
Jeff is one of the students who were awarded.
傑夫是那些獲獎的學生中的一個。
Jeff is the only one of the students who was awarded.
傑夫是那些學生中唯一一個獲獎的。
- what不可引導定語從句
what本身是“複合關係代詞”，等同於“先行詞+關係代詞( which/that)”。因此不能用what引導定語從句。
Tell me what is happening. =Tell me all that is happening.
告訴我正發生著什麼事。
She isn’t what she used to be. =She isn’t the one (that) she used to be.
她不是過去的她了。
She did all what she could to help the poor (x)
She did all ( that ) she could to help the poor. (V)
She did what she could to help the poor. (V)
地盡其所能幫助窮人。
3 .定語從句中關係代詞與關係副詞的選用
(1)用關係代詞還是用關係副詞完全取決於從句中的謂語動詞。及物動詞後面無賓語，就必須要用關係代詞，而不及物動詞則要用關係副詞。
This is the village where/in which he was born.
這就是他出生的村莊。
This is the mountain village (which/that) he visited last year.
這就是他去年來過的村莊。
I will never forget the days when/during which I spent thesummer holidays in the countryside.
我將水遠不會忘記我在鄉下過暑假時的那些日子。
I will never forget the days (which/ that) I spent in the countryside.
我將水遠不會忘記我在鄉下度過的日子。
(2)看定語從句中缺少什麼成分以確定是選用關係代詞還是關係副詞。如果定語從句中缺少主語、賓語、定語或表語，則這個關係詞應是關係代詞; 如果定語從句中缺少時間狀語、地點狀語或原因狀語，那麼我們應該選用關係副詞。
Beijing is the place where(=in which)I was born.
北京是我的出生地。( where在從句中作地點狀語)
Hele is the man whom/that I saw yesterday.
他就是我昨天看到的那個人。(whom/that在從句中作賓語)
