在表示猜测的时候，一定是用完成时态吗？做练习的时候，全都是must have been, can't have been，没有其它的吗？

情态动词(modals)可以对现在、将来、和过去发生的事情的猜测~~~可以用在很多不同的时态中，如现在时，将来时、进行时。。。。。。

Present Time (如果他每天都很准时上班，突然今天没有来，你可以猜测):

He must/may/could be sick.

Past Time:

He must have been sick.

Future Time: （对将来的事做出预测，天气预报是很典型的例子）

We might get some snow tomorrow.

Progressive forms of modal

All of the lights in Ann's room are turned off. She must be sleeping. (right now)

当然不是啊，知识对过去情形的猜测才用must/can't have done对现在和将来则用must/can't be/do/be doing.

知识对过去情形的猜测才用must/can't have done对现在和将来则用must/can't be/do/be doing.

It must have rained yesterday昨天一定下过雨了。must 情态动词1。必须。 you must study hard.2。禁止。You mustn't walk on the grass.3。可能性。表示推测。“一定”1) must do/be. 对现在的动作或状态推测。He must be Tom.他一定是Tom.2) must be doing.对现在的正在进行的动作推测。Tom must be sleeping now. Tom 现在一定在睡觉。3) must have done.对过去的动作或状态推测。Tom must have stayed up yesterday.Tom 昨天一定熬夜了。否定形式 can not /could not

不同的情况用不同时态。

very different

我觉得仔细体会下新概念英语第一册中相关内容即大致明白了。