他本来要参加会议的，但是生病了。（因此没参加）
have to 和must在表示“必须”这个意思时是很接近的，但也有一定的区别。
1. 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 becau the train is to leave at 10:05.（客观需要）
注意下面两句的含义：kinko
Though she didn't need to, she must go.
虽然她不用去，她却偏要去。
You don't have to come again, but you must.
你不必再来，可是你执意要来。
机制英文2. 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 dismisd.
务必马上去见经理，不然你会被开除的。（急迫的事情）
3. 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：
terrifyI have to (must) leave now.
We'll have to buy another TV t.opera什么意思
They had to put off the sports meet becau of the bad weather.
He said that the work must be finished within two weeks.
4. 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.
There must be something wrong with the machine.
5. "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。
must not表示禁止，是说话人强有力的劝告或命令，意为“一定要，不准”。因此，在回答由must引导的问题时，如果是否定的回答，表示“不必，没有必要”，不能用mustn't，而要用needn't或don't have to.试比较：
You must not drive fast.你不能开快车。（路险或有速度限制）
You needn't drive fast.（时间充裕）
You must not tell others.（警告）
You needn't tell others.（没有必要）
needs为副词，相当于necessarily, of necessity. must needs和needs must均可表示“必须，必定，不得不”，这层意义上可以通用；但must needs还可表示“偏偏，偏要”，含有讥讽、不满的意思，而needs must则一般无这层意思。例如：
I must needs go there now.
我现在非到那里去不可。（可用needs must）
Needs must when the devil drives.
情势所迫不得不那样。
She must needs go away when I want her.
我正需要她时，她偏偏离开了。（不可用needs must）gotten
The telephone must needs ring when I went to bed.
我上床睡觉时偏偏有人来电话。（不可用needs must）
need指主语的主观特需情况而使之必要。need既可以作助动词，又可以作行为动词。作助动词时，need没有人称和数的变化，后接不带to的不定式（动词原形），否定式为needn't.作行动动词时，need同别的行为动词一样，有人称和数的变化，后接带to的不定式，否定式要在前面加don't（doesn't, didn't），疑问句用do (does, did) 提问。例如：
There need be no hurry, need there?
He need never know. (=He never needs to know.)
I need only add a few words. (= I only need to add a few words.)
比较：
Father nt me the book, so I didn't need to write to him for it.
父亲把那本书寄来了，所以我不必为此给他写信了。（信没写）
Father nt me the book, so I needn't have written to him for it.美国队长字幕下载
父亲把那本书寄来了，所以我本不该为此再写信给他的。（信已写）
She didn't need to come.她不必来的。（实际也没来）
She needn't have come.她本来不必来的。（但却来了）
Note:
作助动词用时，need只用于疑问句中和否定句中，在肯定句中常用must, have to, should, ought to等。例如：
Need you do it right now? I needn't, but he must.
She doesn't need to be told. She has already known it.
（客观情况使得告诉她没有必要：她已经知道）
She needn't be told. We should keep it cret from her.
（主观上不愿告诉）
1. ought没有词形变化，通用于所有人称，可以用于现在时，过去时和将来时，同带to的不定式连用，否定式为ought not to (oughtn't)，疑问式把ought放在主语前。例如：
He knew he ought not to tell her that.
