Must 和 Have to
must (无词形变化)
（1）。表”必须”,\"应该”，\"务必” must not（mustn't)表”禁止\"，\"不许”，\"不准”,\"不可以”。 Everyone must attend the lecture。 The work must be finished as soon as possible.
You mustn't lend it to others.
注；对must问句的回答。 —Must I go there now？
—Yes， you must.
—No ,you ①needn’t. ②need not ③don’t have(got) to ④don't need to.
2.must表推测”一定”，\"必定\一般只用于肯定句，亦即用于疑问句和否定不表揣测。
(1）对现在He must be at home now. You must be hungry after the long walk.
The Chinese language must have the largest number of speakers。
（2)对进行时。 He must be sleeping now（at that time).
(3)对过去时或完成时。
It must have rained last night. He must have fallen asleep(then）。
He must have gone to Beijing yesterday。 She must have been young when she got married .
注意其反意问句.
3 .must表不可避免的倾向. All men must die。
4 . must 表”偏偏\不巧” My car must break down.我的车偏偏坏了。
have to \"不得不”
1。 与must的区别 (1)must表说话人的主观看法，而have to表客观需要。
比较：I have to stop smoking .(外界压力，客观情况使然）
I must stop smoking。(主观认为）
2。must只有一种形式而have to有更多的形式have to， has to ,had to ，will have to 等等。
I thought I must go there.
3.have to 的疑问,否定均须借助于do。 Does he have to go there now ?
不说Has he to go there now？
He has to go there now， doesn’t he？ The work has to be finished before 10 o'clock..
4。must与had to的差别： had to还可以表示业已完成或实现的动作（即；动作确实做了）。
I had to stay at home last night。
Can
1。表\"能力\"I can lift the stone。 I can speak English while he can’t。 Who can play the violin? He got up early yesterday so that he could catch the early bus。
could 表过去的能力.
注;1。can 与be able to表能力时的区别：
① 形式上;can有两种形式can 与could,而be able to有更多的形式.
is(are)able to, was able to ,, has been able ， will be able to , would be able to 等。
be able to 只表能力而can 还可表”可能性”，\"惊异\"，\"许可”等。
② could与was able to表过去能力的差别:均表过去的能力，但was（were)able to还可表业已成功的行为（即：动作确实做了).
2．表”许可”You can go now. You can use my dictionary. You can have a rest.
—Can I smoke here?
-No， you can’t。
注:用could代替can在疑问句中,语气客气委婉,但在时间上还指现在.Could you lend me your bike? Could you tell me how to get to the station？
注意:回答由could引起的问句,仍用can 而不用could。
—Could you wait a few more minutes？
-Yes， I can。
3。表猜测”可能\一般用于疑问句和否定句.
-Can the news be true？ -It can’t be true. Where can he be？
注:（1)can't表\"不可能\"，语气断然否定。
(2) could在宾语从句中表过去的可能性，其它情况一般还表示现在的可能性只是语气比can委婉和更加不肯定。 I thought he could come 。
(3） can 亦可用于肯定句表可能，但表示的是逻辑推理上的可能性(或理论上的),非主观臆断。There can be no noise on the moon because of sound being carried by the air .
（4） can（could)…have done或can(could）…have been(done）用于疑问句或疑问句表对过去时或完成时的揣测。
What can have happened to him ？ He can not have read the book. He can't have been to Beijing.
4.表”惊异””惊讶\"：How can you be so impolite？ How can you say that？
May
表\"允许””许可”\"允诺\"（征询对方许可）。
You may go now . May I use your bike? You may keep the book for 2 weeks 。
注意:（1)其否定形式may not表示不可以，但更常用must not来代替may not表示\"不可以\"，\"禁止\"。 -May I watch TV after supper？
—Yes, you may.（No， you mustn’t .或No, you may not。或No， you’d better not。)
(2)与can的比较：may比can更正式。May I know your name? May I （he, we…)…。？
×May you….？ （May you have a good journey! 此处表祝愿）
(3）.might代替may用于疑问句，更客气礼貌些,回答时仍用may。
—Might I use your telephone？ —Yes, you may .
(4)might not 不表示”不允许\"
2．表\"可能”。（1）用于肯定句和否定句（因为May I…？表征询许可）。
The news may be true 。 He may not be at home now。 I think he may come today（tomorrow）.
However fast you may run, you can’t catch the fast train。
(2）用might语气更加不肯定，但指现在或将来，只有在宾语从句中might do（be）才表示过去可能性。 He might come today(tomorrow）。 She might have some fever 。
I thought you might like something to read, so I brought some books for you 。
She said she might not be at home。. I might be busy tomorrow.
（3）may not 与cannot的区别： may not”可能不\\"。
比较The news may not be true。
The news can't be true 。
（4) may（might)(not）+have done(been)表对过去或完成时的揣测.
Something may have happened to him . He may have been to Beijing .
3。表祝愿May you be happy! May you have a pleasant journey!
综合性补充；（1)could have done和might have done 还可以用于虚拟语气而can（may)have done 只用于揣测。
（2）表揣测用于反意问句的情况。
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