在現代英語中，have to 中不定式和疑問句式既可以按照助動詞的變化規則構成，也可以按照行為動詞的變化規則構成。

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換用，但有時表示不同的含義。比較：

He has to report to the headquarters every two days.（例行公事，習慣動作）

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

另外，作“有”解時，have got和have通常是可以換用的，have got更口語化，但有時表示不同的含義。比較：

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，是說話人強有力的勸告或命令，意為“一定要，不準”。因此，在回答由must引導的問題時，如果是否定的回答，表示“不必，沒有必要”，不能用mustn't，而要用needn't或don't have to.試比較：

You must not drive fast.你不能開快車。（路險或有速度限制）

You needn't drive fast.（時間充裕）

You must not tell others.（警告）

You needn't tell others.（沒有必要）

needs為副詞，相當于necessarily, of necessity. must needs和needs must均可表示“必須，必定，不得不”，這層意義上可以通用；但must needs還可表示“偏偏，偏要”，含有譏諷、不滿的意思，而needs must則一般無這層意思。例如：

I must needs go there now.

我現在非到那里去不可。（可用needs must）

Needs must when the devil drives.

情勢所迫不得不那樣。

She must needs go away when I want her.

我正需要她時，她偏偏離開了。（不可用needs must）

The telephone must needs ring when I went to bed.

我上床睡覺時偏偏有人來電話。（不可用needs must）

need指主語的主觀特需情況而使之必要。need既可以作助動詞，又可以作行為動詞。作助動詞時，need沒有人稱和數的變化，后接不帶to的不定式（動詞原形），否定式為needn't.作行動動詞時，need同別的行為動詞一樣，有人稱和數的變化，后接帶to的不定式，否定式要在前面加don't（doesn't, didn't），疑問句用do (does, did) 提問。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.)

比較：

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

父親把那本書寄來了，所以我不必為此給他寫信了。（信沒寫）

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

父親把那本書寄來了，所以我本不該為此再寫信給他的。（信已寫）

She didn't need to come.她不必來的。（實際也沒來）

She needn't have come.她本來不必來的。（但卻來了）

Note:

作助動詞用時，need只用于疑問句中和否定句中，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.

（客觀情況使得告訴她沒有必要：她已經知道）

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

（主觀上不愿告訴）

完成下面五道選擇題目：

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