引导定语从句的有：关系代词who(whom, whose)，which, that和关系副词when, where, why等。例如：

The girl who had lunch with me yesterday is my girl friend.昨天与我共进午餐的那个女孩是我的女朋友。(girl是先行词，who是关系代词)

The book that(which)I am reading is very interesting.我正在看的这本书非常有趣。(book是先行词，that是关系代词)

现略谈一谈关系代词和关系副词的作用和用法，引导定语从句的关系代词和关系副词在先行词和定语从句之间起联系作用，它们可以作定语从句的一个成分：

一、关系代词可以作定语从句的主语、宾语或定语。

1.who指人，是主格，在定语从句中作主语；whom是宾格，在定语从句中作宾语；whose是所有格，在定语从句中作定语。例如：

(1)This is Uncle Li who mended computers for us.这就是为我们修电脑的李叔叔。

The student who(that)is playing with a yoyo is only seven.正在玩溜溜球的那个学生仅仅七岁。

(2) This is the man(whom)I met in Australian last month.这就是我上个月在澳大利亚遇见的那个人。

The soldier whom you want to see has already come.你要见的那名士兵已经来了。

(3)There is nobody here whose name is Gu Jianfei.这儿没有名叫顾剑飞的人。

My aunt whose photo I showed you yesterday will come to see us this evening.我姨妈今晚要来看我们，她的照片昨天我给你看过。

2.which指物，在定语从句中作主语或宾语。例如：

(1)Take the pencil which is lying on the desk.把课桌上的那支铅笔拿去。

We visited the Jade Buddha Temple, which is in the northwestern part of Shanghai.我们参观了玉佛寺，它在上海西北边。

(2)The factory which we visited yesterday is very famous in the world.我们昨天参观的那家工厂是全球闻名的。

The money which they found in the street is mine.他们在街上找到的钱是我的。

3.that指物，有时也指人，在定语从句中作主语或宾语。指物时，它的用法和which大致相同。例如：

(1)The comrade that you saw is a combat hero.你见到的那个同志是个战斗英雄。(that作宾语，指人)

Is this the pen that you were looking for?你要找的钢笔是这一支吗?(that作宾语，指物)

(2)The man that called me last night was killed this morning.昨夜给我打电话的那个人今早给人杀了。(that作主语，指人)

There is no difficulty that cannot be overcome.没有不能克服的困难。(that作主语，指物)

[注1]that和which都指事物时，一般可以通用。但在下列情况下必须用that，而不能用which:

1先行词为all，everything, nothing, something, anything, much little, few, none, the one等时。例如：

Say all(that)you know.把你知道的全部讲出来。

Is there anything(that)I can do for you now?现在需要我为你做点什么吗?

This book contains much little that is useful.这本书中有很多(没有多少)有用的东西。

I mean the one that was bought yesterday.我指的是昨天买的那个。

2先行词为数词或被序数词(含last)修饰时。例如：

This is the first film(that)I have seen since I came here.这是我到这里以来所看的第一部电影。

The last place they visited was the Summer Palace.他们最后参观的地方是颐和园。

Look at those presents. You can see the two that you gave me.看那些礼物，你能看见你送我的那两个吗?

3先行词被形容词的最高级或the very, the only等所修饰时。例如：

This is the biggest apple(that)I have ever eaten.这是我所吃到的苹果中最大的一个。

She is the very thief(that)the policeman is looking for.她就是警察正在寻找的那个小偷。

Mary is the only friend(that)I have in Australia.玛丽是我在澳大利亚唯一的朋友。

4先行词中既有人又有物时。例如：Peter and his car that disappeared mysteriously in London in 1987 appeared mysteriously in New York in 1993.彼得和他的小轿车1987年在伦敦神秘地失踪，后来于1993年又神秘地出现在纽约。

We were deeply impressed by the teachers and schools that we had visited.我们访问过的老师和参观过的学校给我们留下了深刻的印象。

5先行词在主句中作表语，或关系代词在从句中作表语时。例如：

This is a dictionary that will help you a lot.这是本对你很有帮助的字典。

Don't cheat me. I'm no longer the boy that I was three years ago.别骗我了，我再也不是三年前的我了。

[注2]在下列情况下不宜用that:

1.关系代词前有介词时。例如：

This is the house in which I lived ten years ago.这就是我十年前住过的房子。

2.先行词本身是that时。例如：

What was that which he said?他说了些什么?

[注3]whom,which和that在定语从句中作宾语时，特别是在口语中，往往可以省略。例如：

The man(that)you got the news from is a friend of mine.告诉你这个消息的人是我的朋友。

Is that the car(which)you want to buy?那就是你所要买的汽车吗?

This is the little boy(whom)Comrade Li saved yesterday.这就是李同志昨天救的那个小男孩。

二、关系代词whom, which在定语从句里作介词的宾语时，介词一般放在先行词与关系代词之间。例如：

I want to make some friends from whom I can learn a lot.我想交几个朋友，从他们那儿我可以学许多东西。

但是，有时介词也可以放在宾语从句的后面，特别是在省略了关系代词时，介词必须放在定语从句的后面。例如：

The room which(that)we live in is very bright.

The room we live in is very bright.我们住的那间房间很明亮。

三、关系代词which有时指整个前面一句话。这时关系代词前面有逗号，定语从句是附加的说明；which的意思相当于and this。例如：

Tom said he didn't know anything about Kate, which was a lie.汤姆说他对凯特的情况一无所知，这是撒谎。

The worker didn't do any work，which made his boss very angry.那个工人什么活也没干，这令老板非常生气。

四、关系副词where, when和why在定语从句中分别作地点状语、时间状语和原因状语。

1.where指地点，在定语从句中作状语，相当于in等＋which。例如：

This is the workshop where(=in which)I work.这是我工作的车间。

A booking office is a place where(=in which)tickets are sold.售票处就是卖票的地方。

2.when指时间，在定语从句中作状语。例如：

He came at a time when we needed help.他在我们需要人帮忙的时候来到了。

3.why指原因、理由，在定语从句中作原因状语。例如：

The reason(why)he changed his mind is not clear.他改变主意的原因尚不清楚。下面再略谈一谈限制性定语从句和非限制性定语从句。限制性定语从句是先行词不可缺少的定语，如果省去，主语的意思就会不完整或不明确。这种从句和主句之间不能用逗号分开。非限制性定语从句，只是对先行词的附加说明，如果省去，主句的意思仍然很清楚，它和主句之间常用逗号分开。试比较下列句子：

(1)I have a brother who is a doctor.我有一个当医生的兄弟。(意思是我还有其他兄弟)(限制性定语从句)

(2)I have a brother, who is a doctor.我有一个兄弟，是个医生。(意思是我只有一个兄弟)(非限制定语从句)

[注1]在非限制性定语从句里不宜用关系代词that。因此，凡是that所引导的定语从句，一般都是限制性定语从句。例如：

误：She gave me some flowers, that were very beautiful.

正：She gave me some flowers, which were very beautiful.(非限制性定语从句)

正：She  gave  me  some  flowers  that(which)were very beautiful.(限制性定语从句)

[注2]非限制性定语从句在口语里很少用，尤其是在对话里，经常是采用简单句或并列句。例如：“I have a brother, who is a doctor.”这句话，在口语里总是说：“I have a brother; he is a doctor.”或说：“I have a brother, and he is a doctor.”