I' m sure he must have been sleeping at the moment 的反义疑问句

I' m sure he must have been sleeping at the moment,wasn't he?
must have done表示推测的时候,它后面的反义疑问句可以有两个时态,一个是过去式,一个是完成时,这里应该指“当时”有没睡着,用过去
再问： at the moment 是此刻 为什么不是hasn't he
再答： 如果是“此刻”的话前面就不用must have been 了吧，那应该是must be sleeping ，所以这里的at the moment只能选“那时”的意思
再问： 是过去式为什么不用didn't he