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.
