1. I dare to swim across this river.
2. He doesn\\'t dare (to) answer.
3. He needs to finish his homework today.
五、 shall, should
1) shall 用于第一人称，征求对方的意见 。
What shall we do this evening?
2) shall 用于第二、三人称，表示说话人给对方的命令、警告、允诺或威胁 。
1. You shall fail if you don\\'t work hard.(警告)
2. He shall have the book when I finish it.(允诺)
3. He shall be punished.(威胁)
六、 will, would
1) 表示请求、建议等，would更委婉 。
Will / Would you pass me the ball, please?
2) 表示意志、愿望和决心 。
1. I will never do that again.
2. They asked him if he would go abroad.
3) would表示过去反复发生的动作或某种倾向 。would表示过去习惯时比used to正式，且没有“现已无此习惯”的含义 。
1. During the vacation, he would visit me every other day.
2. The wound would not heal.
4) 表示估计和猜想 。
It would be about ten o\\'clock when she left home.
七、 should, ought to
1) should, ought to表示“应该”，ought to表示义务或责任，比should语气重 。
1. I should help her because she is in trouble.
2. You ought to take care of the baby.
2) 表示劝告、建议和命令 。should, ought to可通用，但在疑问句中常用should 。
1. You should / ought to go to class right away.
【情态动词的三种用法 情态动词的用法总结】2. Should I open the window?
3) 表示推测
should , ought to (客观推测), must(主观推测) 。
1.He must be home by now. (断定他已到家)
2.He ought to/should be home by now.(不太肯定)
3. This is where the oil must be.(直爽)
4. This is where the oil ought to/should be.(含蓄)
八、 情态动词+不定式完成式（have done）
1) can / could + have done在肯定句中表示“本来可以做而实际上能做某事”，是虚拟语气；在疑问句或否定句中表示对过去行为的怀疑或不肯定, 表示推测 。
1. You could have done better, but you didn\\'t try your best. （虚拟语气）
2. He can\\'t have been to that town.（推测）
3. Can he have got the book?（推测）
2) may / might +不定式完成式（have done）
表示对过去行为的推测 。不能用于疑问句中，没有虚拟语气的用法 。Might所表示的可能性比may小 。
1. He may not have finished the work .
2. If we had taken the other road, we might have arrived earlier.
3）must +不定式完成式（have done）
用于肯定句中，表示对过去行为的推测 。意为“一定、想必” 。其疑问、否定形式用can，can\\'t代替 。参看1) can / could + have done表示推测 。
1. You must have seen the film Titanic.
2. He must have been to Shanghai.
4）should +不定式完成式（have done）
用于肯定句中，表示对过去行为的推测 。
He should have finished the work by now 。
表示“本应该做而实际上没有做某事”，其否定式表示某种行为本不该发生却发生了 。可以与ought to +不定式完成式（have done）互换 。
1. You ought to / should have helped him. (but you didn\\'t.)
2. She shouldn\\'t have taken away my measuring tape, for I wanted to use it.
5) needn\\'t +不定式完成式（have done）
表示“本来不必做而实际上做了某事” 。
You needn\\'t have watered the flowers, for it is going to rain.
6) will +不定式完成式（have done）
主要用于第二、三人称，表示对已完成的动作或事态的推测 。
He will have arrived by now.

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