对啊.如果不用形式主语,举个例子
（That we wait for him right now ）is impossible.我们现在不可能等他的.
这个句子很明显,that 从句是主语从句.
即使把句子改成用it作形式主语
It is impossible that we wait for him right now.
that引导的那个句子仍然是主语从句.