在现代英语中，have to 中不定式和疑问句式既可以按照助动词的变化规则构成，也可以按照行为动词的变化规则构成。

Have you to finish the work before supper?

Do you have to finish the work before supper?

have got to 常可同have to换用，但有时表示不同的含义。比较：

He has to report to the headquarters every two days.（例行公事，习惯动作）

He has got to report to the headquarters every two days.（一道指示或命令，必得每天报告一次）

另外，作“有”解时，have got和have通常是可以换用的，have got更口语化，但有时表示不同的含义。比较：

The man has a blind eye.那人有一只眼瞎了。

The man has got a blue eye.那人有一只眼被打青了。

must not表示禁止，是说话人强有力的劝告或命令，意为“一定要，不准”。因此，在回答由must引导的问题时，如果是否定的回答，表示“不必，没有必要”，不能用mustn't，而要用needn't或don't have to.试比较：

You must not drive fast.你不能开快车。（路险或有速度限制）

You needn't drive fast.（时间充裕）

You must not tell others.（警告）

You needn't tell others.（没有必要）

needs为副词，相当于necessarily, of necessity. must needs和needs must均可表示“必须，必定，不得不”，这层意义上可以通用；但must needs还可表示“偏偏，偏要”，含有讥讽、不满的意思，而needs must则一般无这层意思。例如：

I must needs go there now.

我现在非到那里去不可。（可用needs must）

Needs must when the devil drives.

情势所迫不得不那样。

She must needs go away when I want her.

我正需要她时，她偏偏离开了。（不可用needs must）

The telephone must needs ring when I went to bed.

我上床睡觉时偏偏有人来电话。（不可用needs must）

need指主语的主观特需情况而使之必要。need既可以作助动词，又可以作行为动词。作助动词时，need没有人称和数的变化，后接不带to的不定式（动词原形），否定式为needn't.作行动动词时，need同别的行为动词一样，有人称和数的变化，后接带to的不定式，否定式要在前面加don't（doesn't, didn't），疑问句用do (does, did) 提问。例如：

There need be no hurry, need there?

He need never know. (=He never needs to know.)

I need only add a few words. (= I only need to add a few words.)

比较：

Father sent me the book, so I didn't need to write to him for it.

父亲把那本书寄来了，所以我不必为此给他写信了。（信没写）

Father sent me the book, so I needn't have written to him for it.

父亲把那本书寄来了，所以我本不该为此再写信给他的。（信已写）

She didn't need to come.她不必来的。（实际也没来）

She needn't have come.她本来不必来的。（但却来了）

Note:

作助动词用时，need只用于疑问句中和否定句中，在肯定句中常用must, have to, should, ought to等。例如：

Need you do it right now? I needn't, but he must.

She doesn't need to be told. She has already known it.

（客观情况使得告诉她没有必要：她已经知道）

She needn't be told. We should keep it secret from her.

（主观上不愿告诉）

完成下面五道选择题目：

1. You__ Jim anything about it. It was none of his business.

A needn't have told B needn't tell C mustn't have told D mustn't tell

2.As it turned out to be a small houseparty, we __ so formally.

A need not have dressed up B must not have dressed up C did not need to dress up D must not dress up

3.We could__ him with a detached house when he came, but he had specifically asked for a small flt.

A provide B have provided C not provide D not have provided

4.He__ the 8:20 bus becasue he didn't leave home till 8:25.

A couldn't have caught B ought to have caught C shouldn't have caught D must not have caught

5."She must be in the dormitory now."

"No, she __ be there. I saw her in the classroom a minute ago."

A mustn't B can't C couldn't D wouldn't

答案：

AABAB