What he wants is a piece of paper.
It is believed that he can solve the problem.
I don't know how to solve the problem.
Do you know where he lives?
The problem is who can help me.
This is why I came here.
I have no idea where he went.
I heard the news that he would come.
同位语从句用that引导，常跟在fact, idea, news, promise, thought, message, hope, belief, doubt等词后，that在从句中不作任何成分。
在复合句中，修饰某一名词或代词，或修饰整个主句的从句叫做定语从句，引导定语从句的关系代词有who, whom, whose, which, that, as,和关系副词when, where, why。
a. 先行词有all, everything等不定代词时，如，
Everything (that) he did is wrong.
b. 先行词被all, every, no, some, any, little, much等修饰时，如，
I'll read all the books (that) you lend me.
This is the first letter (that) the boy has written.
d. 先行词被the only, the very, the same, the last修饰时，如
He is the very man (that) I'm looking for.
This is the book about which we have talked a lot.
The book, which he gave me yesterday, is very interesting.
This is the room where I worked.
This is the room which I stayed in.
I remembered the day when we lived there.
I remembered the day that I spent there.
as 可以放于句首，而which 不可以
As you know, he is good at English.
three of them 和three of which
I have a lot of books, three of which are in Russian.
I have a lot of books and three of them are in Russian.
例1、 _______ I accept the gift or refuse it is none of your business.
A. If B. Whether C. Even if D. No matter when
例2、The way _______ these comrades look at problems is wrong.
A. where B. in that C.X D. with which
解析：该题答案为C，先行词是way，定语从句中用that或 in which来引导或不填。
例3， ____a long time since I saw you last time.
A．It was B. It is C. It had been D. It can be
解析：该题答案为B，It is +时间数+ since引导的从句是一个句型，意为"从…时候以来过了多久了。"