Might I ask for a photograph of your baby? (比May I …更客气一些)
She can't be serious.
A more suitable book can't be found. (It is not possible to find a more suitable book.)
He couldn't (can't) be over fifty.
should, ought to: 表示应该做的事, ought to比should口气稍重一些.
You should (ought to) do as he says.
You shouldn't (oughtn't to) talk like that.
但这两者间也有一些差别, 在表示责任, 义务等该做的事情时, 常用ought to, 在表示某件事宜于做时, 多用should, 在下面的句子中这两个词就不宜换用:
You are his father. You ought to take care of him.
We should not use too many big words in our everyday speech.
情态动词可以和动词的进行式构成谓语, 表示”应该正在……”, “想必正在……”这类意思:
Why should we be sitting here doing nothing?
This isn't what I ought to be doing.
She might still be thinking about the question you raised.
They must (may) be waiting for us, let’s hurry up.
They can't be using the room now.
情态动词有时和动词的完成形式构成谓语, 表示”应当已经……”, “想必已经……”这类意思:
I should have thought of that.
They shouldn't have left so soon.
She must have arrived by now.
You needn't have told them that.
Where can (could) he have gone?
He can't have finished the work so soon.
He may not have achieved all his aims. But his effort is praise-worthy.
We ought to have give you more help.
They are sweating all over. They must have been working in the fields.
They may have been discussing the problem this morning.
You should have been waiting for us. Why haven't you?
She couldn't have been swimming all day.