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.”