情态动词的语法(3)

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。

21.5 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。