3）must表必然性 You must catch cold if you don't put on more clothes.。

4) must 表示"一定要（语气很强的邀请）" 和"非得（嫌弃对方固执）"

You must stay for dinner.

Must you play the piano at such a late hour?

2. have to 和 don't have to/needn't

1) have to表示"必须"，"不得不"，在这个意义上与must很接近，但must表示的是说话人的主观看法，而have to 表示的却是客观需要。

You have to work hard to make a living.

You must do what I tell you.

2）have to 有更多的时态形式：现在时，过去时，将来时等，而must只有现在时形式。

We had to be there at 8:00.

I shall have to go to school tomorrow.。

3）don't have to 表示"不必"，"不一定要", 意思是说某事可以做也可以不做，在这个意思上needn't 则表示根本没必要做。

You don't have to win all the matches to be a top club.

We will do a thorough cleaning to the house but we needn't clean the roof.

3. ought to/ ought not to

1）ought没有人称或时态的变化，后跟带to的不定式。常译作"应该"，"应当"等，和should同义，只是口气稍重一些。其否定式为ought not to (oughtn't to )，疑问式为Ought I /you to...?

Such things ought not to be done.

2）表可能性,意思是"按常理会应该... "

Mary ought to be home by now.

3）表示没有履行过去的义务时，用"ought to +完成时。

You ought to have told me about this earlier. (But you didn't)

四 我的练习

一、情态动词填空

1. Jack described his father, who a brave boy many years ago, as a strong-willed man.

2. Mark have hurried. After driving at top speed, he arrived half an hour early.

3. You ______ park here! It's an emergency exit.

4. You buy a gift, but you can if you want to.