大學英語六級情態動詞語法精要

　　情態動詞:
　　may/might表示允許和可能:
　　允許: 詢問或說明一件事可不可以做.
　　May I trouble you with a question?
　　You may take this seat if you like.
　　He asked if he might glance through my album.
　　You might as well speak your mind. ***比may…顯得婉轉一些***
　　可能: 表示一件事或許發生***或是某種情況可能會存在***.
　　You may walk ten miles without seeing a house.
　　She was afraid they might not like the idea.
　　A bad thing might be turned to good account.
　　might 表示請求:
　　Might I ask for a photograph of your baby? ***比May I …更客氣一些***
　　can’t, couldn’t表示否定的推測:
　　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: 表示應該做的事, ought to比should口氣稍重一些.
　　You should ***ought to*** do as he says.
　　You shouldn’t ***oughtn’t to*** talk like that.
　　但這兩者間也有一些差別, 在表示責任, 義務等該做的事情時, 常用ought to, 在表示某件事宜於做時, 多用should, 在下面的句子中這兩個詞就不宜換用:
　　You are his father. You ought to take care of him.
　　We should not use too many big words in our everyday speech.
　　will, would
　　shall, should表示意願
　　情態動詞後接進行式, 完成式和完成進行式:
　　情態動詞可以和動詞的進行式構成謂語, 表示”應該正在…”, “想必正在…”這類意思:
　　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.
　　情態動詞有時和動詞的完成形式構成謂語, 表示”應當已經…”, “想必已經…”這類意思:
　　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.
　　情態動詞間或也可以和一個動詞的完成進行式構成謂語,表示”應當一直在…”, “想必一直在…”這類意思:
　　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.