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.”

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