2. 关系代词在定语从句中作宾语时，可以省略。
Behind one door was a hungry tiger the guards had put there.
The tiger would immediately jump upon the poor man the guards had brought to the arena.
3. whose只用作定语,可以用来指人或物; 若指物，它还可以同of which互换, 例如：
They rushed over to help the man whose car had broken down.
Please pass me the book whose cover is green.
关系副词引导的定语从句
关系副词可代替的先行词是时间、地点或理由的名词，在从句中作状语。
1. 关系副词when, where, why的含义相当于介词+ which结构，因此常常和介词+ which 结构交替使用，例如：
There are occasions when one must yield.
Beijing is the place where I was born.
Is this the reason why he refused our offer?
限制性和非限制性定语从句
例如：
1)This is the house which we bought last month.
The house, which we bought last month, is very nice.。
2) 当先行词是专有名词或物主代词和指示代词所修饰时，其后的定语从句通常是非限制性的，例如：
Charles Smith, who was my former teacher, retired last year.
My house, which I bought last year, has got a lovely garden.
This novel, which I have read three times, is very touching.
3) 非限制性定语从句还能将整个主句作为先行词, 对其进行修饰, 这时从句谓语动词要用第三人称单数，例如：
He seems not to have grasped what I meant, which greatly upsets me.
Liquid water changes to vapor, which is called evaporation.
说明：关系代词that和关系副词why不能引导非限制性定语从句。
b. 名词性从句
在句子中起名词作用的句子叫名词性从句。名词性从句的功能相当于名词词组,它能在复合句中作主语、宾语、表语、同位语等。根据它在句中不同的语法功能，名词性从句分为主语从句、宾语从句、表语从句和同位语从句。在名词性从句中，连接词that既没有实际意义，又不充当任何成分，只起引导从句的作用, 引导宾语从句时经常省去。
一、主语从句 ：在复合句中作主句的主语。为保持句子结构平衡，主语从句通常被置于句末，而用it作形式主语。
That light travels in straight line is known to all.
When the plane is to take off hasnt been announced.
It was my fault that I had him play football yesterday afternoon.
It is important that we should go to the school to talk with the teacher.
It is still unknown which team will win the match.
二、宾语从句 ：主要作及物动词的宾语，有时置于介词或形容词之后。
He said his father had come back from the United States.
He asked how much I paid for the violin.
No one can be sure what man will look like in a million years.
如果宾语从句带有补足语，通常用it作形式宾语，而把宾语从句置于补足语之后。
I think it necessary that we should pay close attention to SARS.
We all feel it our duty that we should serve the people heart and soul.