定語 從句由關係詞（關係代詞、關係副詞）引導，關係代詞、關係副詞位於定語從句句首。
高中定語從句的詳細講解
一 定語：修飾名詞或代詞的詞、短語或從句稱為定語。漢語中常用‘……的’表示。 定語主要由形容詞擔任。此外名詞、代詞、數詞、副詞、介詞短語以及動詞不定式（短語）、分詞也可以做定語。
a beautiful city; a bag full of money.注意：漢語的定語無論多長都放在被修飾詞的前面，而英語中的定語則不然，是一個詞時，放在被修飾詞的前面， 若是兩個以上的片語、短語或從句則放在被修飾詞的後面，
thirty women teachers ; his father; the girl in red. the boy from America
Our monitor is always the first student to enter the classroom.
falling leaves; fallen leaves; the boy playing basketball; the book bought by my mother; a reading room; a swimming pool
He is the man who you are looking for.
二：定義及相關術語
1．定語從句：修飾某一名詞或代詞的從句叫定語從句。定語從句一般緊跟在它所修飾的先行詞之後。
2．先行詞：被定語從句修飾的名詞或代詞叫先行詞。
3．關係詞：引導定語從句的詞叫關係詞。關係詞有關係代詞和關係副詞。關係代詞有that, which, who, whom, whose, as等；關係副詞有when, where, why.
1. I have an apple. An apple is red.
I have an apple that/which is red. ↑ ↑
先行詞 關係代詞
2.I like some friends. Some friends like sports.
I like friends who like sports.
↑ ↑
先行詞 關係代詞
3.I like music. The music is quiet.
I like music that/which is quiet.
↑ ↑
先行詞 關係代詞
關係詞通常有下列三個作用：A、引導定語從句，連線主從句；B、代替先行詞；C、在定語從句中擔當一個成分。
關係代詞引導的定語從句
1．who 指人，在定語從句中作主語。
The boys who are playing football are from Class One. Those who want to go to the museum must be at the school gate at 7 tomorrow morning. Yesterday I helped an old man who had lost his way. That is the teacher who teaches us physics. 2．whom 指人，在定語從句中做賓語，常可省略。
Mr Liu is the person ( whom ) you talked about on the bus. Li Ming is just the boy ( whom ) I want to see. The professor ( whom ) you are waiting for has come. The girl ( whom ) the teacher often praises is our monitor.
注意：關係代詞whom 在口語或非正式文體中常可用who 來代替，也可省略。
The man ( whom / who )you met just now is my old friend.
3．Which 指物，在定語從句中做主語或賓語，做賓語時常可省略。
Football is a game which is liked by most boys. He likes to read books which are written by foreign writers. The house which is by the lake looks nice. This is the pen ( which ) he bought yesterday. The film ( which ) they went to see last night was not interesting at all. 。
4．That 即可指人，也可指物.。在定語從句中作主語或賓語，作賓語時常可省略。
The number of people that / who come to visit this city each year reaches one million.
Where is the man that / whom I saw this morning? The person that /whom you introduced to me is very kind.
The season that / which comes after spring is summer.
Yesterday I received a letter that / which came from Australia.
5．Whose 即可指人，也可指物，在定語從句中做定語。
I visited a scientist whose name is known all over the country. He has a friend whose father is a doctor.
I once lived in the house whose roof has fallen in. 注意：指物時，常用下列結構來代替：
The classroom whose door is broken will soon be repaired.
The classroom the door of which is broken will soon be repaired.
Do you like the book whose cover is yellow?
Do you like the book the cover of which is yellow?
（一）限制性定語從句中只能用that 引導定語從句的`情況
1．當先行詞是everything, anything, nothing (something 除外), all, none, few, little, some等不定代詞時，或當先行詞受every, any, all, some, no, little, few, much等不定代詞修飾時。如：
Have you taken down everything that Mr. Li said?
There seems to be nothing that is impossible to him in the world.
All that can be done has been done. 。
There is little that I can do for you.
He stayed in the library and looked up any information that they needed.
注意：當先行詞指人時，偶爾也可用關係代詞who，如：
Any man that / who has a sense of duty won’t do such a thing. All the guests that / who were invited to her wedding were important people.
2．當先行詞被序數詞修飾時。如：
The first place that they visited in London was the Big Ben.
3．當先行詞被形容詞最高階修飾時。如：
This is the best film that I have ever seen.
4．當先行詞被the very, the only修飾時。如：
This is the very dictionary that I want to buy.
After the fire in his house, the old car is the only thing that he owns.
注意：當先行詞指人時，偶爾也可用關係代詞who。如：
Wang Hua is the only person in our school who will attend the meeting.
5．當先行詞前面有who, which等疑問代詞時。如：
Who is the man that is standing by the gate?
Which is the T-shirt that fits me most?
6．當先行詞為人與物時。如：
They talked about the persons and things that they remembered at school
（二）關係副詞引導的定語從句
1． when 指時間，在定語從句中作時間狀語。當先行詞是表時間的名詞，比如time, day, week, month, year等；且定語從句中缺時間狀語時用when. 若先行詞是表時間的名詞，定語從句中缺主語或賓語時，用that/which引導。
I still remember the day when I first came to this school.
The time when we got together finally arrived.
Do you remember the years when he lived in the countryside with his grandpare？Do you remember the days（that/which） we spent together last year? I will never forget the days when we spent our holidays together. I will never forget the days that / which we spent together.
2． where 指地點，在定語從句中作地點狀語。若先行詞是表地點的名詞，比如park, city, farm, place等；且定語從句中缺地點狀語時用where. 若先行詞是表地點的名詞，定語從句中缺主語或賓語時，用that/which引導。
Shanghai is the city where I was born. 上海是我出生的城市。
The house where I lived ten years ago has been pulled down. I visited the farm where a lot of cows were raised . Shanghai is the city (which/that) I want to visit.
I know a place where we can have a picnic. I know a place which / that is famous for its beautiful natural scenery.
3． why 指原因，在定語從句中作原因狀語。若先行詞是reason; 且定語從句中缺原因狀語時，用why引導。若先行詞是reason，定語從句中缺主語或賓語，用which/that引導。
Please tell me the reason why you missed the plane.
I don’t know the reason why he looks unhappy today.
I didn’t believe the reason (that/which) he explained to me
注意：關係副詞引導的定語從句經常可以用“介詞+關係代詞”引導的定語從句來表示。如：
From the years when / in which he was going to primary school in the country he had known what he wanted to be when he grew up. Great changes are taking place in the city where / in which they live. The reason why / for which he refused the invitation is quite clear.
（三）“介詞+關係代詞which/whom”引導的定語從句
關係代詞在定語從句中作介詞賓語時，從句常常由“介詞+關係代詞”引出。
The school (which / that) he once studied in is very famous.
The school in which he once studied is very famous.
Tomorrow I’ll bring here the magazine (which / that) you asked for.
Tomorrow I’ll bring here the magazine for which you asked. This is the boy (whom / who / that) I played tennis with yesterday.
This is the boy with whom I played tennis with yesterday. We’ll go to hear the famous singer (whom / who / that) we have often talked about.
We’ll go to hear the famous singer about whom we have often talked. The manager whose company I work in pays much attention to improving our working conditions.
The manager in whose company I work pays much attention to improving our working conditions.
注意：
１． 含有介詞的固定短語動詞一般不拆開，介詞仍放在短語動詞的後面。如：look for, look after, take care of 等。
This is the watch (which / that) I am looking for. (正)
This is the watch for which I am looking . (誤)
The babies (whom / who / that) the nurse is looking after are very healthy. (正)
The babies after whom the nurse is looking are very healthy. （誤）
２．若介詞放在關係代詞前，關係代詞指人時只可用whom，不可用who, that；關係代詞指物時只可用which，不可用that。
The man with whom you talked just now is my neighbour. (正) The man with that / who you talked just now is my neighbour. (誤)
The plane in which we flew to Canada was really comfortable. (正) The plane in that we flew to Canada was really comfortable. (誤)
３．“介詞+關係代詞”前還可有some, any, none, all, both, neither, many, most, each, few等代詞或者數詞。如：
He loves his parents deeply, both of whom are very kind to him.
In the basket there are quite many apples, some of which have gone bad.
There are forty students in our class in all, most of whom are from big cities.
Up to now, he has written ten stories, three of which are about country life. 注意事項:如何判斷關係代詞和關係副詞
