A computer can't think for itself; it must be told what to do.

　　电脑不具备独立思考的能力。

　　I should have been there, but I could not find the time.

　　You can come any time you like.

　　Anybody can make mistakes.

★表达某事实际发生的可能性时用may, could或might等。

We may spend our summer holidays in Hangzhou this year.

"Are you coming to Jeff's party?" "I'm not sure. I might go to the concert instead."

★could/might 用来表示请求允许或许可时,语气比can/may委婉,但在答语中不能用could/might.如:

--Could I use your telephone?

--Yes, you can.

★当表示过去成功地做了某事时,要用was/were able to, 而不能用could.如:

　　The fire spread through the hotel very quickly but everyone was able to get out.

　　大火很快蔓延了整个旅馆, 但全体人员都脱离了危险.

　　It was a holiday and the children were able to go to the seaside.

★mustn't 表"禁止,不准";cannot 表"不可能; 拒绝"; need not表"不必要"; may not 表"可能不".

　　You mustn't play with the knife, you may hurt yourself.

"Will you stay for lunch?" " Sorry, I can't. My brother is coming to see me."

"Is John coming by train?" " He should, but he mat not. He likes driving his car."

" Shall I tell John about it?" "No, you needn't. I've told him already."

★may用于征求对方许可， shall用于征求对方的意见或指示。

　　May I have a look? 我能看一下吗？

　　Shall I have a look? 要我看一下吗？

★shall用来征求对方意见时用于第一、三人称。当表示说话人的命令、警告、强制、允诺、威胁或决心等时用于第二、三人称。

　　Shall she go to the concert with us this evening? （征求意见）

　　You shall go to the front at once. （命令）

　　Don't worry. You shall get the answer this very afternoon. （允诺）

　　He shall be sorry one day. I tell you. （警告）

Nothing shall stop us from fighting against pollution. （决心）