先看选择题：
_____ is known to everybody, the moon travels round the earth once every month.
A. It B. As
C. That D. What
此题很容易误选 A，认为此处应填一个形式主语，况且 It is (well) known to … 也是一个常用句型，读起来也很上口，但是选A错了。同学们仔细想想看，假若选 A，空格处填一个形式主语(it)，那么其后就应有一个真正的主语，但事实上没有。但是若句中的逗号改成 that，则只能选 A，因为这样一来，that 便成为一个引导主语从句的引导词。此题的最佳答案是 B，as 引导的是一个非限制性定语从句，可置于主句前(常译为“正如”)或主句后(常译为“这一点”)，句意为“正如大家所知，月球每月绕地球转一周”。
1. it is known to sb that从句
It是形式主语（代替后面的动词不定式或者主语从句），that从句是真正的主语，即从句是真正的主语，如：
It Is known to us that the earth revolves around the sun.
众所周知，地球围绕太阳公转。
It is known to all that man can't live without air or water.
众所周知，人离开空气或水就不能生存。
It is known to us all that water is the source of life.
众所周知，水是生命之源。
It is known to us all that the computer is the most important invention in the 20th century.
众所周知，计算机是在第二十世纪最重要的发明。
It is known to all that exercises are good for health.
众所周知，锻炼有益于身体健康。
It is known to us all that Liu Xiang is a man of iron.
众所周知，刘翔是个意志坚强的人。
有时接how等从句
It is known to us how he became a writer.
我们都知晓他是如何成为一名作家的。
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.
2. as is known to sb结构，as 引导的是一个非限制性定语从句，可置于主句前(常译为“正如”)或主句后(常译为“这一点”)，如：
As is known to all, China is a developing country, not a developed one.
众所周知，中国是个发展中国家，而不是发达国家。
As is known to all, paper was first made in China.
众所周知，纸是中国人发明的。
As is known to us, knowledge is power.
中所周知，知识就是力量！
As is known to all, anyone who breaks the rules should be punished.
众所周知,任何违反规则的人都应该受到惩罚。
As it is known to us all, reading has a great impact on our whole life.
我们大家都知道，阅读对我们的一生有巨大的影响。
As is known to us, heavy smoking ruins our health.
众所周知，吸烟过量毁坏身体健康。
The earth revolves around the sun，which Is known to us.
地球围绕太阳公转，这个我们都知道。
当非限定定语从句which Is known to us前置到句首，which必须换成as，转换成as is known to us，所以形成；
As is known to us， the earth revolves around the sun.
3. as is known to us不能作为独立句，后面必须接主句；
As is known to us, the Chinese are a industrious and gallant nation.
众所周知，中国人民是勤劳勇敢的民族。
4. ，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 .
我们大家都知道他明天要来我们学校。
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!
众所周知，地球绕太阳转！