Question

as is known,it is known不同用法？

Answer 1

As is known to all 是由as引导的定语从句的形式 It is known to all 是由It做形式主语而引导的句子
如 As is known to all, the 2008 Olympic Games was a great success. 总所周知，2008奥运会取得巨大的成功。
此时AS是作为非限制性定语从句的引导词
It is known to all that the 2008 Olympic Games was a great success. 总所周知，2008奥运会取得巨大的成功。此时it只是个形式主语，真正的主语是由后面that引导的句子。