4. 同位语从句，在句中作某些名词的同位语，对前面的名词进一步说明。
用在下列名词 fact, news, idea, hope, belief, thought, doubt. truth, order, suggestion, word etc.
(1)There is no doubt that he will come.
There is doubt whether he will come.
Word came that Napoleon would come to inspect them.
There is no possibility that….
(2)that引导的同位语从句与定语从句的区别：
在同位语从句中：that不可以省略，不作成分;定语从句的关系代词that在定语从句作成分，作宾语时可以省略
We are interested in the news that some foreigners would visit our school.
We are interested in the news that he told us.
when, where引导的同位语从句与定语从句的区别：
在定语从句中，when, where前有分别表示时间、地点的先行词；而在同位语从句中无此对应关系
They have no idea at all where he has gone.(同位语从句)
Go and get your coat. It’s in the place where you left it.(定语从句)

5. 疑问词-ever既可以引导名词性从句，也可以引导让步状语从句，而no matter+疑问词只能引导让步状语从句
No matter where I go, my heart is towards China.
I believe whatever he says. Whatever he says, I will never believe him.
who与 whoever的区别
who是“谁”的意思，表示具有疑问，whoever相当于anyone who“无论谁”
Whoever leaves the room last ought to turn off the lights.
Can you tell me who that gentleman is?

6. 当名词性从句缺少主语、宾语或表语时，引导词通常用what。另外，有时具有感叹意义all that, anything that或 the thing(s) that
What we can’t get seems better than what we have.
Tom thought what a beautiful girl Mary was.

7. 注意区分it 作形式主语的主语从句与强调句
强调句句型It is/was+被强调部分+that, 去掉It is/was, that，句子仍然成立。
It is natural that they should have different views. 主语从句
It is only lately that he had had a family himself. 强调句
What was it that he wanted I don’t know what it was that he wanted.
It was in the hotel where he stayed that we discussed the serious problem

8. 注意it 作形式主语的主语从句和as引导非限制性定语从句引导的不同
It is known to all that the earth is round.
As is known to all, the earth is round.