mary couldn't have received my letter, or she have replied the letter last week.
有了具体的last week 为什么还要用完成时态?
为什么用couldn't ?能直接用 mary haven't received my letter 么?
还有一个句子请帮忙分析一下
nancy is supposed to have finished her chemical experiment at least two weeks ago.
为什么用完成时态