have to的用法
1)must表示一种主观的需要，而have to 表示一种客观的需要，意思是“不得不”。 如：
I have to attend an important meeting this afternoon.
今天下午我不得不参加一个重要的会议。
Mother is out, so I have to look after the shop.
妈妈不在家，因此我不得不照看商店。
2)have to 的否定形式是don't have to, 相当于needn't。如：
They don't have to buy a computer at present.
他们目前没有必要买电脑。
3)have to 与have got to 常可以互换。
4)have to 比较强调客观需要，表示因客观环境或事态促使而不得不做某事;must强调主观看法，表示主观上认为有必要做某事。例如：
I must learn another language.
(主观想法: I want to)
I have to learn another language.
(客观需要：身为一个外交官)
You must be back before 10 o'clock.
(叮嘱或命令)
You have to be back before 10 o'clock because the train is to leave at 10:05.(客观需要)
5) have to多表示义务或习惯动作;must则用于表示一种重要或急迫的事情。例如：
We have to care for the young.(义务)
She has to be at the office before eight every day.(习惯)
You must go to the manager at once, or you'll be dismissed.
务必马上去见经理，不然你会被开除的。(急迫的事情)
6) have to可用于不同的时态;而must一般只用于现在时(但也可以表示将来的情况，在间接引语中也可以表示过去时)。例如：
I have to (must) leave now.
We'll have to buy another TV set.
They had to put off the sports meet because of the bad weather.
He said that the work must be finished within two weeks.
7) must还可以表示一种推断和揣测，而have to则不能。例如：
This must be Jin's pen.
8)"must+动词完成式"可以用来表示对过去情况的揣测，而have to则不能。例如：
She must have read the book sometime in the past, or she couldn't have answered the question so well.
她一定在过去某个时候读过这本书，不然，这个问题她不会回答得这么好的。
Note:
在现代英语中，have to 中不定式和疑问句式既可以按照助动词的变化规则构成，也可以按照行为动词的变化规则构成。例如：
Have you to finish the work before supper?
Do you have to finish the work before supper?
have got to 常可同have to换用，但有时表示不同的含义。比较：
He has to report to the headquarters every two days.(例行公事，习惯动作)
He has got to report to the headquarters every two days.(一道指示或命令，必得每天报告一次)
另外，作“有”解时，have got和have通常是可以换用的，have got更口语化，但有时表示不同的含义。比较：
The man has a blind eye.那人有一只眼瞎了。
The man has got a blue eye.那人有一只眼被打青了。
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