It is known to all that the earth is round.这句话属于什么句型?

问题描述:

It is known to all that the earth is round.这句话属于什么句型?
It is known to all that the earth is round.
这句话属于什么句型?
为什么?

众所周知,地球是圆的.
it是形式主语.作为形式主语的it并无实际语义,只是为满足语法上的需要,避免句子头重脚轻,它代替的是句子的逻辑主语.这句话真正的主语是"the earth is round"