三. must 和 have to 的用法

1. 表示“必须、必要”。

You must come on time.

当must 引出的问句时， 若是否定的回答，不能用mustn’t，而用needn’t或don’t have to.

— Must we hand in our exercise our books today?

— Yes, you must. (No, needn’t/you don’t have to.)

2. “must be + 表语”的结构表示推测，它的否定或疑问形式用can 代替must。如：

He must be our new teacher.

Can he be our new teacher? He can’t be our new teacher.

※ 附加疑问句要和谓语动词实际时态一致。如：

He must be our new teacher, isn’t he?

3. “must + have +done” 的结构常用在肯定句中，表示对过去或完成的情况推测，它的否定或疑问形式也用can代替must.

He must have finished his work.

※ 附加疑问句也要和谓语动词实际时态一致。如：

He must have finished his work already, hasn’t he?

He must have finished his work at four yesterday, didn’t he?

4. have to 的含义与must相似，两者往往可以互换使用，但have to有各种形式，随have的变化而定，must 与have to有下列几点不同：

1) must表示的是说话人的主观看法，而have to则往往强调客观需要，例如：

The play is not interesting. I really must go now.

I had to work when I was your age.

2) 两者的否定意义大不相同。例如：

You mustn’t go. (你不可以去) You don’t have to go. (你不必去)

3) 询问对方的意见时应用must。

Must I hand in the exercise book before eight?

4) must 表示“偏偏”

Must it rain today? (今天怎么偏偏下雨呢?)

四. dare 和need的用法

1. need表示“需要，必须”，作情态动词时，仅用于疑问句和否定句，在肯定句中一般用must, have to, ought to,或should代替。例如：

You needn’t come so early.

— Need I clean all the room right now? — Yes, you must. (— No, you needn’t.)

注意：needn’t + have +done表示“本来不必要做的事而实际上做了”，例如：

You needn’t have waited for me. 你本来没有必要等我的。

2. dare作情态动词时，主要用于疑问句，否定句和条件句中，一般不用于肯定句，例如：

How dare you take my book without permission?

He dare not speak English before such a crowd, dare he?

3. need和dare作情态动词时，过去时形式与现在形式相同(美国英语中可用dared)。

He was so angry that no one dared say a word.

They said that he needn’t stay there any longer.

4. need和dare常用实义动词，有时态、人称和数的变化。不同的是dare作实义动词时，在肯定句中通常接带to的不定式，在疑问句和否定句中，dare之后的不定式可带to或不带to。例如：

He needs to finish the work before nightfall.

I dare to swim across this river.

He does not dare (to) answer the teacher’s question.

Don’t you dare (to) touch it?

I wonder he dared say that.

5. I dare say是习惯用语，常可写为I daresay，用法有以下几种：

1) “敢于说”，通常接that或what引导的名词性从句。

I dare say that he has stolen Tom’s football. I dare say what I think.

2) “我想;恐怕;大概;也许是”，通常作插入语用，使句子委婉。

I dare say there is something wrong with the machines.

You’re tired, I dare say.

You, I dare say, think otherwise.

3) I dare say往往也作为反语使用。

Oh, you mean to defeat our football team? I dare say you will. (噢!你们想赢我们的足球队?也许可以吧。)