He passes the exam is impossible.把这个句子变成主语从句和强调句!说明差别在哪里

That he has passed the exam is impossible.
It is impossible that he has passed the exam.只能说第二个也是一个主语从句,it是形式主语第一句主语从句怎么没有形式主语it 呢？因为That he has passed the exam 已经放到主语的位置上了，不需要形式主语。