英语语法结构What is know to us all is that …… It is konw to us all

这三种结构的相同点:意思相同,都表示"众所周知/我们大家都知道"
不同点:用法各不相同.
It is known to us all中的It为形式宾语,因此这种结构后必须接一个主语从句来作真正的主语.
这个主语从句可以是that或wh-类连词引导的从句.即:
It is known to us all+that/what/when/where/how从句.
as is known to us all是由as引导的定语从句,
as指代后面或前面的整个主句,因此as is known to us all结构可放句首,也可放句中或句末,
但一定要用逗号与主句隔开.
what is known to us all是一个名词性从句,常用来作主语从句,用于这个结构中:
What is known to us all is that.
例如:It is known to us all that he will visit our school tomorrow .
=As is known to us all ,he will visit our school tomorrow .
=What is known to us all is that he will visit our school tomorrow .
As is known to us all,the earth goes round the sun!
=It is known to us all that the earth goes round the sun!
=What is known to us all is that the earth goes round the sun!
It is known to us all when the meeting will be held .
It is known to us all where the meeting will be held .
It is known to us all why you didn't come to school yesterday.

What is know to us all 是what引导的主语从句，意思是 我们所知道的 ，后面的that引导的是表语从句
第二句it是形式主语，后面的that是真正的主语，这样做事为了句子平衡美观，避免头重脚轻
第三个句子，as引导的是原因状语从句，意思是 正如大家都知道的
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1（what is known to us all) is that
主语 系 表、
2 it is known to us all that
形式主语 谓语 that 引导实际主语
3 as is known to us all, 是as 引导的定语从句，as 翻译成就像