It must have been about two in the morning when I returned home .must have been 是什么语法

问题描述:

It must have been about two in the morning when I returned home .must have been 是什么语法
为什么加been

情态动词加完成时,表示对过去的肯定推测
should have done 表示本应该做,实际没做
为什么加been
因为是be的过去分词,be about 用完成时,就是have been ...