dare, used to, ought to.had better上述两类助动词的共同特征是，在协助主动词构成限定动词词组时，具有作用词的功能：
1）构成否定式：
He didn't go and neither did she. The meeting might not start until 5 o'clock.
2）构成疑问式或附加疑问式：
Must you leave right now? You have been learning French for 5 years, haven't you?
3）构成修辞倒装：
Nowhere can he obtain any information about his sister. Hardly had he arrived when she started complaining. 4）代替限定动词词组：
A: Who can solve this crossword puzzle? B: Tom can. A: Shall I write to him? B: Yes, do.
重点疑难
（一）need和dare的用法
need和dare既可用作情态动词，也可用作实义动词。用作情态动词时，主要用于否定句和疑问句。用作实义动词时，可用于各种句式。
1.用作情态动词
--Need I come? --Yes, you must. --我需要来吗？ --需要。 You needn′t telephone him now. 你现在不必打电话给他。 I don′t think you need worry. 我想你不必发愁。
She dare not go out alone at night. 她晚上不敢一个人出去。 How dare you say I′m unfair? 你竟敢说我不公平？
Not one of them dared mention this. 他们谁也不敢提这件事。 2.用作实义动词
You don′t need to do it yourself. 你不必亲自做这件事。 We need to tell them the news. 我们需要把这消息告诉他们。 The table needs painting (to be painted.). 桌子需要油漆一下。 We should dare to give our own opinion. 我们要敢于提出自己的观点。 He did not dare (to) look up. 他不敢抬头看。
I dare day he′ll come again. 我想他会再来的。 （cI dare say?为固定习语） （二）情态动词后跟完成式和进行式的用法
1.情态动词后跟完成式，表“应当已经??”，“想必已经??”，“本来可以??”等意。
I should have finished the work earlier. 我应当早一点完成这项工作的。 He isn′t here. He must have missed the train. 他还未到，一定是没赶上火车。 Where can (could) he have gone? 他能到那里去了呢？
You may (might) have read about it. 你可能在报上已经读到这件事了。 You could (might) have been more careful. 你本来可以更细心的。 He needn′t have worried about it. 他本不必为此事担心。
There was a lot of fun at yesterday′s party. You ought to have come, but why didn′t you
昨天的聚会非常有意思。你本应该来，为何不来呢？
2. 情态动词后跟进行式，表示“想必正在??”，“可能正在??”，“应当正在”等意。
It′s twelve o′clock. They must be having lunch. 现在是十二点。他们一定正在吃饭。
They may be discussing this problem. 他们可能正在论讨这个问题。 He can′t be telling the truth. 他说的不可能是真话。
She shouldn′t be working like that. She′s still so weak. 她不应当那样干，她身体仍那么虚。
（三）几组词的辨异 1. can 和be able to
1）情态动词can只有两种时态形式，现在式can和过去式could，而be able to有多种时态形式。
Mary can play the piano. She has been able to play it since she was 5. 玛丽会弹钢琴。她五岁起就会弹了。
2）用在过去时中，could经常表示能够做某事，事实上不一定去做，而was∕were able to则表示“过去做成了某事”。在否定句中两者可通用。
He could swim across the English Channel. But he didn′t feel like it that day. 他能游过英吉利海峡，但那天他不想游。
Yesterday I was able to get home before the heavy rain. 昨天我在下大雨前赶到了家里。
2. must和 have to
must表示主观意志，而have to表示由于客观因素不得不做某事。must没有过去式，除在间接引语中可用于表示过去时间，在直接引语中表示过去时间应该用had to代替。
I told her that she must give up smoking. 我叫她必须戒烟。
We had to get everything ready that night. 我们那晚得把一切准备就绪。 3. would和used to
1）used to表示过去与现在或过去某时与后来的情况有不同，而would只表过去的情况。People used to think that the earth was flat.
过去人们认为地球是平的。（现在人们不这么认为。）
She would go out for a walk in the morning when she was in the country. 在乡下时，她总是在早晨去散会儿步。（可能现在仍有散步的习惯。） 2）used to可表示过去的习惯动作和经常的情况，而would只表示过去的习惯动作。
He used to ∕would smoke while writing. 过去他写东西时常抽烟。 She used to be fat. 她过去很胖
专项练习（一）
1. I __________ you, because I thought I must be wrong.
A. dare not ask B. dare not to ask C. dare not asking D. dare to not ask 2. There __________ some flowers in the garden.
A. were used to be B. used to be C. uses to be D. used to be having
3. “__________ I take it out?” “I′m sorry, you __________.”
A. Could ?couldn′t B. Might?might not C. Could?can D. May?can′t
4. You were stupid to climb the tree. You __________ hurt yourself. A. may B. might C. will D. might have 5. You __________ those letters. Why didn′t you ?
A. should post B. should have posted C. must have posted D. ought to post.
6. All the lights are on, the Smiths __________ up.
A. must get B. is getting C. must be getting D. would get 7. He __________ lead a horse to the water but he __________ not make it drink.
A. will?can B. may?can C. may?dares D. dare?can 8. “Need we do this job now?” “Yes, __________.”
A. you need B. you should C. you must D. you can 9. __________ to have lunch with us today?
A. Do you likes B. Would you like C. Will you liked D. Have you liked
10. He said that you __________ watch TV all the evening if you wished. A. may B. must C. can D. might
11. –Is John coming by train? --He should, but he __________ not. He likes driving his car.
A. must B. can C. need D. may 12. Peter __________ come with us tonight, but he isn′t very sure yet. A. must B. can C. may D. will 13. Michael __________ be a policeman, for he′s much too short.
A. needn′t B. can′t C. shouldn′t D. won′t 14. I thought you __________ be hungry, so I have brought you some cakes. A. may B. might C. can D. could 15. I wonder how he __________ that to the teacher.