Question

it is a 抽象名词 that 是什么从句？

Answer 1

It is a good idea that you will go with us.你愿意同我们一起去是一个好主意。

it是形式主语，that you will go with us是主语从句，是真正的主语。

也可以不用形式主语，即：

That you will go with us is a good idea.

Answer 2

it is a 抽象名词 that 是主语从句，it是形式主语，that引导真正的主语从句。