can't be+v must be+v怎样表示,过去式呢,过去完成的呢?

can't be+v must be+v怎样表示,过去式呢,过去完成的呢?
can't be+v must be+v怎样表示,过去式呢,过去完成的呢?

can't be+v must be+v怎样表示,过去式呢,过去完成的呢?
couldn't be 不可能不能够
couldn't have been 过去不可能做过如I couldn't have said that .我不可能说过那样的话
must be 一定(Somebody is knocking at the door.It must be Tom有人在 敲门,一定是Tom)
must have been过去一定做过
(It must have been rained last night,beacuse the ground is wet.昨晚一定下雨了因为地面湿了)

过去式
couldn't be
must be

过去完成时
couldn't have been
must have been

couldn't be
couldn't have been
must be
must have been