情态动词的进行式
情态动词+行为动词进行式(即情态动词+ be + v-ing形式)，表示推测或评论某动作现在是否正在进行。例如：
The light in his study is still on; he must be working now.他书房的灯还亮着，想必他现在还在工作。
She shouldn’t be working like that. She is still very weak.她不应该这样干,她身体还这样弱.
He can’t be telling the truth. 他说的不可能是真话.
情态动词的完成进行式
情态动词+行为动词完成进行式(即情态动词+ have been + v-ing 形式)，表示推测或评论过去某动作是否正在进行或一直在进行。例如：
They are sweating all over. They must have been working in the fields.他们浑身是汗，准是在地里一直干活来着。
They may have been discussing this suggestion all the morning.今天上午他们可能一直在讨论这个建议来着。