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同学你好~
通常说As is known to all，这里as作主语。
as is often the case，这里as就是主语，不必加it了。

as 引导非限制性定语从句，代替整个主句，对其进行说明。常用于as we all know, as is known to all, as is it, as is said above, as is often the case, as is reported in the newspaper等句式中，通常均由逗号将其与主句隔开。as 有“正如.....”,  “就像....”之意。
As we all know, the earth is round. 众所周知，地球是圆的。
As is known to the United States, Mark Twain is a great American writer. 正如美国人都知道的，马克吐温是一位伟大的美国作家。

可以在这里看看as引导的非限定性定语从句
http://www.hjenglish.com/new/p1209734/

祝同学学习进步~