题目内容：

It’s known to all that.把it换成what,that后面为什么要加上is.

优质解答

原句it是做形式主语,真正的主语是后面that引导的从句,相当与that...is known to all.
it改成What之后,就是What引导的主语从句了,这时that前要加is,变成What is known to all is that...这里that引导第二个系动词is后的表语从句.