can't be+v must be+v怎样表示,过去式呢,过去完成的呢?

网友回答

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)
======以下答案可供参考======
供参考答案1:
过去式couldn't be
must be
过去完成时couldn't have been
must have been
供参考答案2:
couldn't be
couldn't have been
must be
must have been