It might not be known to the public that the President was i

主语从句。原句等于：That the President was in poor health might not be known to the public。that引导主语从句。it might not be known (to the public )that the President was in poor health。it是形式主语,真正的主语是后面的that从句。为了不使句子看的“头重脚轻”所以把that从句放在后面）It's well-known to all that he is the richest man。(That he is the richest man is well-known)