Question

1.___ is known to us all is that China is a developing country. A. Which B. what C. as D.

1.___ is known to us all is that China is a developing country. A. Which B. what C. as D. It2.___ is known to us that China is a developing country. A. Which B. As C. What D. It答案：1B 2D。1是表从，为什么不能用It？2是主从，为什么不用what？

Answer 1

1题如果用it,得说.It is known to us all that China is a developing country.
2题用的就是我改写第一句的句型
看下面你就明白了：
What is known to us all(主语从句）is that China is a developing country
.It(形式主语）is known to us that China is a developing country（that引导主语从句）

追问

It，what都能引导名词性从句，怎么用？

追答

It 不可以，它不是连词。
what可以。见下例：
What he has said(主语从句）sounds reasonable.
This is what I bought yesterday(表语从句）。
Let me look at what you have pained(宾语从句）

追问

1因为后面有一个谓语is了，如果填¡t代词作主语又有一个iS谓语，所以填引导词可以这样理解吗？

追答

你已经大致明白了。
如果你用it 作形式主语的话，这就是主句中的谓语部分“is known to us all ”，后面的that China is a developing country就是真正主语了. 这样其前的is 就不知道是什么了哈哈。明白？

Answer 2

1. 如果用It, it 是形式主语，那么后面就是that 引导的真正主语。而这个句子后面有个系动词is，说明前面只能填一个主语从句，what is known to us all, “我们都知道的是....”
2. 而第二句话就是It（形式主语）+谓语+真正主语（从句）的结构。

追问

It，what都能引导名词性从句，怎么用？

追答

it 是个代词，不是引导从句的词，it在句子中只能做主语、宾语
what 是个引导词，可以引导主语从句、宾语从句

追问

1因为后面有一个谓语is了，如果填¡t代词作主语又有一个iS谓语，所以填引导词可以这样理解吗？

追答

对啊，楼主正解！

Answer 3

第一题，后面那个is是真真的谓语，所以前边就是名词性短语，因而用What。
第二题，缺少主语，因而填It