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.
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