情態+動詞進行/完成進行時態
情態動詞也可以和動詞進行形式或完成進行形式構成謂語：表示應當正在，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.
情態動詞間或也可以和一個動詞的完成進行形式構成謂語，表示應當一直在，想必一直在這類意思。
【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.
