改为 need not do 或 don’t have to do 。
a. You must get up at six tomorrow morning.
You need not get up at six tomorrow morning. / You don’t have to get up at six tomorrow morning. （3）回答以must提问的句子，可以如下形式进行肯定或否定形式作答。
Yes, you must.
a. Must we clean all the rooms? No, you don’t have to.
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No, you needn’t.
6、must 和have to的区别
四、ought to
1、表示“应该”（与should同义，只是语气稍重一些, ought to 强调责任、义务），无人称和时态变化。 a. You are his father. You ought to take care of him.
b. You ought not to smoke here. c. Ought you to smoke so much?
五、shall, should
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1、在疑问句中，用于第一、第三人称，表示征求对方的意见。 a. Shall we do this evening? b. Shall I do it for you?
c. Shall he come in or wait outside? = Do you want him to …?
2、用于第二、第三人称，表示说话人给对方的命令、警告、允诺或威胁。
a. You shall fail if you don’t work harder.
b. He shall be punished.
c. You shall have the book after I finish reading it.
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3、用在法律、条约、规章、预言等文件中，无论主语人称如何，一律用shall，表示义务、规定。 a. Each citizen shall carry his identification card when travelling. b. Death is certain to all; all shall die. 4、should表示劝告、建议、命令，其同义词是ought to。在疑问句中，通常用should代替ought to。 a. You should go to class right away. b. Should I open the window?
c. The book is good and you should buy one.
5、should表示义务，意指应该做，
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