must

当表示“经过努力才得以做成功某事”时应用be able to，不能用Can。如：

He was able to go to the party yesterday evening in spite of the heavy rain.

must必须，应当

①You must get to the airport before nine o'clock. 你必须在九点以前到达飞机场。

must的用法

1.表示必须、应该(否定形式用needn't或don't have to )。例如:

-Must I finish the work before dark?

-No, you needn't / don't have to. You may leave it for tomorrow.

2.must not (mustn't)表示强烈禁止。例如:

You mustn't get to school late.

3.表示推测,只用在肯定句中,'一定'、'想必'之意。其否定意义'不可能',用can't be形式。例如:

-The light in her room is still on; she must be at home now.

-She can't be at home. I saw her out just ten minutes ago. She forgot to turn off her light.

4.must和have to的区别:

must表示说话人的主观看法;而have to强调客观情况迫使人不得不……;must只能表示现在'必须',表示过去的事情要用had to;have to有时态、人称和数的变化。must和have to的否定意思不同。比较:

You mustn't do that.(你千万不能做那件事。)

You don't have to do that.(你不必做那件事情。)

四、 need的用法

need作情态动词时表示'需要',主要用于否定句和疑问句。例如:

-Need we finish the work today?

-Yes, you must. / No, you needn't.

must not 表示禁止，一定不要这样做。例句:You must not come.你一定不要来。can not 表示（1）不可能 （2)没有能力做=be able to例句:(1)He has gone abroad,it can not be him.他去国外了，那个人不可能是他。

（2）I can not play the piano

must 是必须做

have to是不得不做，带有一种主观的不愿意

must not是禁止

can not是不能，强调超过能力所及的范围

must 表示主观上的必须。What must i do?

have to 强调客观上的不得不。 I have to go now