what is known to us是what引導的名詞性從句，what 在從句中作主語，從句與主句之間沒有逗號；
as is known to all是as引導的非限制性定語從句，as在從句中作主語，從句與主句之間需加逗號，例如：
What is known to us is that the earth is an ellipse我們都知道地球是橢圓形的（本句中what引導主語從句，只能放在句首）
As is known to all, the eaeth is an ellipse.
或者：The earth is an ellipse, as is known to all.所有人都知道，地球是橢圓形的
（本句中as引導非限制性定語從句，在從句中作is known的主語，指代the eath is sn ellipse，as引導的非限制性定語從句可以放在句首，也可以放在主句之後，用逗號與主句隔開）
what is known to us是what引導的名詞性從句，what 在從句中作主語，從句與主句之間沒有逗號；
as is known to all是as引導的非限制性定語從句，as在從句中作主語，從句與主句之間需加逗號，例如：
What is known to us is that the earth is an ellipse我們都知道地球是橢圓形的（本句中what引導主語從句，只能放在句首）
As is known to all, the eaeth is an ellipse.
或者：The earth is an ellipse, as is known to all.所有人都知道，地球是橢圓形的
（本句中as引導非限制性定語從句，在從句中作is known的主語，指代the eath is sn ellipse，as引導的非限制性定語從句可以放在句首，也可以放在主句之後，用逗號與主句隔開）