逍遥右脑 2010-05-07 21:55
比较have to和must
1) 两词都是'必须'的意思，have to 表示客观的需要， must 表示说话人主观上的看法，既主观上的必要。
My brother was very ill, so I had to call the doctor in the middle of the night. 我弟弟病得很厉害，我只得半夜里把医生请来。(客观上需要做这件事)
He said that they must work hard. 他说他们必须努力工作。（主观上要做这件事）
2) have to有人称、数、时态的变化，而must只有一种形式。但must 可用于间接引语中表示过去的必要或义务。
He had to look after his sister yesterday.
3） 在否定结构中： don't have to 表示"不必"
mustn't 表示"禁止"，
You don't have to tell him about it. 你不一定要把此事告诉他。
You mustn't tell him about it. 你一定不要把这件事告诉他。
must表示推测
1) must用在肯定句中表示较有把握的推测，意为"一定"。
2) must表对现在的状态或现在正发生的事情的推测时， must 后面通常接系动词be 的原形或行为动词的进行式。
You have worked hard all day.You must be tired. 你辛苦干一整天，一定累了。（对现在情况的推测判断）
He must be working in his office. 他一定在办公室工作呢。
比较：
He must be staying there.
他现在肯定呆在那里。
He must stay there.
他必须呆在那。
3) must 表示对已发生的事情的推测时，must 要接完成式。
I didn't hear the phone. I must have been asleep. 我刚才没有听到电话，我想必是睡着了。
4) must表示对过去某时正发生的事情的推测，must 后面要接不定式的完成进行式。
---Why didn't you answer my phone call?
---Well, I must have been sleeping, so I didn't hear it.
5) 否定推测用 can't。
If Tom didn't leave here until five o'clock, he can't be home yet. 如果汤姆五点才离开这儿，他此时一定还未到家。
表示推测的用法
can, could, may, might, must 皆可表示推测，其用法如下：
1）情态动词+动词原形。
表示对现在或将来的情况的推测，此时动词通常为系动词。
I don't know where she is, she may be in Wuhan.
2）情态动词+动词现在进行时。
表示对现在或将来正在进行的情况进行推测。
At this moment, our teacher must be correcting our exam papers.
这时，我们老师想必在批改试卷。
3）情态动词+动词完成时。
表示对过去情况的推测。
We would have finished this work by the end of next December.
明年十二月底前我们很可能已完成这项工作了。
The road is wet. It must have rained last night.
地是湿的，昨天晚上一定下雨了。
4）情态动词+动词的现在完成进行时。
表示对过去正在发生事情的推测。
Your mother must have been looking for you.
你妈妈一定一直在找你。
5）推测的否定形式，疑问形式用can't, couldn't表示。
Mike can't have found his car, for he came to work by bus this morning.
迈克一定还没有找回他的车，因为早上他是坐公共汽车来上班的。
注意：could, might表示推测时不表示时态，其推测的程度不如 can, may。