It’s known to all that.改成what is known to all is that 中为什么要加is?

看：
It is known to all that A loves B.(主语从句)
=That A loves B is known to all.（主语从句）
= What is known to all is that A loves B.（主语从句加表语从句）
=A loves B,which is known to all （定语从句）
= As is known to all,A loves B.（插入语结构）
= A loving B is known to all.（动名词结构）
明白了吗?把它们背出来吧,你会收益的.顺便问一下,你读几年级?