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，的意思。
【例如】
Why should we be sitting here doing nothing?
She might still be thinking about the question you raised.
He cant be swimming in such weather.
At the moment she may be playing with her schoolmates.
We neednt 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 couldnt have been swimming all day.
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