情态动词
一.can,could,be able to
1.can：表示“能，会” e.g. I can swim but I can’t skate.
could：过去“能够，可能” e.g. It was so dark that we could see nothing. 与“be able to do” 区别 “be able to do”强调成功做某事
2.can：表示“允许，许可”，常译为“可以”＝permission
Could 委婉语气e.g.：Can/could I use your dictionary? --Yes, you can.
3.can表示客观，理论可能性e.g.：Man cannot live without air.
4.用于疑问句，否定句，表推测。Can’t be 不可能 惊异怀疑不相信的态度e.g. I believe her. She can’t be such a careless girl. How can you be so careless?
5. can’t ?too/enough 再?也不为过
e.g. You can’t be careful enough when you across the road. Could it be possible?×May/Might表可能性用于肯否
二.must,have to
1.must主观e.g. I must finish my homework.
have to客观 e.g. I have to finish my homework.
2.must:表示肯定的，有把握的推测
e.g. He must be at home, for he called me from three months ago.
3.偏要if you must do something.
4.must的回答e.g. Must she go now?
--Yes, she must/No, she needn’t/No, she don’t.
三.should, ought
1. Should:义务或责任. e.g. We should help the person who needs help.
2.竟然，常与how，why连用 e.g. I don’t know why you should think I did it.
3.根据常规经验做出的判断，想必 e.g. 20 minutes have past, the bus should come at any moment.
四.will，would
1.will表示请求建议（用于第二人称） would语气委婉
e.g. Will/Would you please go with me?
2.习惯性，倾向现在wille.g. Fish will die out of water. 过去would e.g. Whenever I was in trouble ，he would help me.
3.意志愿望决心e.g. I will never do it again.
与“used to do”区别，used to do暗指现在不做
五.Shall
1.一，三人称疑问句表示征求对方的意见或请求.shall we go now ？
2.二，三人称陈述句，表示命令，警告，允诺，威胁等语气
e.g. You shall be punished.
3.条约，规定，法律等文件中表义务，某种规定
e.g. Nobody shall leave the classroom during the exam.
六.may，might，unlikely
1.may①译为可能也许 She may come, but I am not very sure.
②表许可 May/Might I take the book out？ May文体比can 正式 ③表祝愿 May you succeed
Might 更婉转语气比may弱
七.情态动词表推测
1.不同时间推测 must do现在/must be doing正在进行/must have done过去
2.must肯定句 can/could否定句，疑问句
may/might 肯定句，否定句，表可能性推测
→must have done一定
→can’t(couldn’t)have done不可能 may not可能不
may(might) have done本可能（虚拟）
八.情态动词＋have done表虚拟
Could have done 本能够，强调能力
Should（shouldn’t） have done本（不）应该
Ought to have done本应该
Needn’t have done （无肯定）本没有必要而实际做了
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