对于形容词我们已十分熟悉了，如 a goo d book， 形容词 good 用来修饰书 book。
我们也可以用一个句子来修饰名词，这种句子叫做形容词性从句，它起修饰名词的作用，又被叫做定语从句The attributive clause。但有一点不同的是这个从句不是像形容词那样放于名词前，而是放在名词之后。它所修饰的名词又被叫作先行词，如 Do you know the scientist who gave us the talk this afternoon 这句中的主句是 Do you know the scientist 你知道那位科学家吗而 who gave us the talk this afternoon 他今天下午给我们作的报告。是定语从句。所以这两句话合为一体即是你认识今天下午给我们作报告的那位科学家吗这里 scientist 叫作先行词，而 who 叫作定语从句的引导词。
who 在定语从句中起主语的作用， who 的数与它的先行词相同。又如 You must do everything that I do 这里先行词是 everything， 而 that I do 是定语从句，此句应译为你必须作我所作的一切。
that 叫作定语从句的引导词，在句中作 do 的宾语。引导定语从句的引导词有关系代词 that， which， who， whom， whose 和关系副词 when ， where， why， how。不论关系代词还是关系副词，都应放于先行词和定语从句之间，起联系作用，但它们都要在定语从句中起语法作用，充当一个成份。如关系代词在定语从句中不是作主语便是作宾语，而关系副词则是作状语。我们先来看关系代词的用法。
① that 的先行词可以是人也可以是物。如 A plane is a machine that can fly 这里先行词是 machine 而 that 是关 系代词，在定语从句中作主语。这句译为飞机是一种会飞的机器。又如 I like the book th at you lent me yesterday 这里先行词是 book， 关系代词用 that， 它在定语从句中作 lend 借的宾语。要注意的是关系代词在定语从句中作宾语时可以省略，即 I like the book you lent me yesterday ② which 关系代词的先行词只能是物。它在定语从句中作主语或宾语，如 The book shop is a shop which sells books 这里 shop 是先行词， which 在从句中作主语。
又如 The book which I read last night was wonderful 这里主句是 The book was wonderful 而定语从句是修饰主句的主语 book， 即我昨晚读的那本书，which 在定语从句中作 read 的宾语，可以省略。
③ who， whom， whose who 在定语从句中作主语， whom 是 who 的宾格，在定语从句中作宾语，而 whose 则是形容词性物主代词，在从句中作定语，如The man who visited our school yesterday is an American friend 昨天参观我们学校的人是一位美国朋友。
Who 在定语从句中作主语。又如 Whos that woman whom you just talked to 你刚才与之谈话的那个女人是谁而 whom 作定语从句中介词 to 的宾语，可以省略，而在现代英语中，句首的 whom 也常常可用 who代替。
This is our classmate， Mary， whose home is not far from our school 这是我们的同学玛丽，她的家离我们学校不远。为了便于理解，我们来看看是如何将两句话并为一句话的。
１． I saw the man. He closed the door I saw the man who that closed the door 2． The girl is happy She won the race The girl who won the race is happy 3． The students are from China They sit in the front row The students who sit in the front row are from China 要注意的是先行词是 students 则 who 的数也应看作复数。
4． We are studying sentences They contain adjective dause We are studying sentences that which contain adjective dause 5． The taxi driver was friendly He took me to the airport The taxi driver who took me to the airport was friendly 6． The book was good I read it The book that I read was good The book I read was good 7． The people were very nice We visited them yesterday Th e people we visited yesterday were very nice 8． The man called the police His wallet was stolen The man whose wallet was stolen called the police 9． I come from a country Its history goes back thousands of years I come from a country whose history goes back thousands of years 10． I have to call the man I picked up his umbrella after the meeting I have to call the man whose umbrella I picked up after the meeting 关系代词 whom， which 在定语从句中作介词宾语时，可以和介词一起放于先行词与定语从句 之间，有时为了关系紧凑也可以将 whom 与 which 与先行词紧挨着书写，而将介词置于定语从句的后面，如 That was the room in which we h ad lived for ten years 或可以写作 That was the room which we had lived in for ten years He was the man whomwho you were looking for 要注意的是此句的关系代词 whom 可以用主格取代，而 look for 是短语动词也不可将 for 放于定语从句之前。that 作关系代词作介词宾语时，不能紧跟介词，而只能将介词置于定语从句的后面。如 The man that we were talking about has come to our school 这时不可用 about that 请看下面例句 1． The meeting was interesting I went to it The meeting that I went to was interesting 2． The man was very kind I talked to him yesterday The man who I talked to yesterday was very kind 3． I must thank the people I got a present from him I must thank the people who I got a present from 4． The picture was beautiful She was looking at it The picture that which she was looking at was beautiful 5． The man is standing over there I told you about him The man who I told you about is standing over there 除关系代词外，还有关系副词， when， where， why， 其中 when用来指时间，在定语从句中作时间状语。如 I never forget the day when I first came to the Great Wall 而 where 则指地点，如 This is the house where the old man lives 请看下面例句 1． The city was beautiful We spent our vacation there The city where we spent our vacation was beautiful 2． That is the restaurant I will meet you there That is the restaurant where I will meet you 3． The town is small I grew up there The town where I grew up is small 4． That is the drawer I keep my newpapers there That is the drawer where I keep my newspapers 5． Monday is the day We will come then Monday is the day When we will came 6． ７∶０５ is the time My plane arrives then ７∶０５ is the time when my plane arrives 7． １９６０ is the year The revolution took place then １９６０ is t