must be /must have been
一定是 那时一定是

he must be a doctor
他一定是位医生

he must have been a doctor
他那时一定是位医生

must这些情态动词后边跟的是动词原形，是BE
但是如果出现这一个问题。。怎么翻译。

他那时一定能抬起这张台
he must have been lift that table

这里been和lift都是动词，是不可能连接在一起的。除非是进行时态..难到中间要+to动词不动式？还是怎样的？
书上教是用进行时态。即he must have been lifting that table.

可以解答一下么。如果这能解答了。之后的can't be/may be那些都会迎刃而解..

你这里只是一个动词be的用法,加上must.

动词可以是现在时(is, do, lift),现在完成时(has/have been, has/habe done, have lifted)现在进行时,将来时(will be, will do, will lift), 现在完成进行时(have been being, have been dooing, have been lifting), 将来完成时(will have been, will have done, will have lifted) ....

must 是必须, 一定,应该, 等等, 后面跟原形.

he must have been lift that table.是个病句.
He must lift that table.
He must have lifted that table.
He must have been lifting that table.

He is a doctor.
He has been a doctor.
He has been being a doctor.(有时加时间段)
He must be a doctor.
He must have been a doctor(注意must 后面用原形have).
He must have been being a doctor for at least 3 years.