情态动词也可以和动词进行形式或完成进行形式构成谓语：表示“应当正在……”，“想必正在……”的意思。
【例如】
Why should we be sitting here doing nothing?
She might still be thinking about the question you raised.
He can t be swimming in such weather.
At the moment she may (might) be playing with her schoolmates.
We needn t be standing here in the rain. We might take shelter in the hut over there.
情态动词间或也可以和一个动词的完成进行形式构成谓语，表示“应当一直在……”，“想必一直在……”这类意思。
【例如】
They are sweating all over. They must have been working in the fields.
They may have been discussing the problem this morning.
She couldn t have been swimming all day.