情態(tài)動詞,顧名思義是用來表示說話人的語氣、態(tài)度和情感的動詞。情態(tài)動詞一般沒有人稱和數(shù)的變化,不能單獨(dú)作謂語,必須和動詞原形一起使用。雖然情態(tài)動詞數(shù)量不多,但是各自表達(dá)的意思卻很豐富,如果不進(jìn)行歸類,容易混淆。為此,作以下幾點(diǎn)講解:
　　
　　can,could和be able to可用來表示能力。be able to可用于各種時態(tài),且was / were able to表示“成功做了某事”。例如:
　　Though the earthquake hit the village, all the villagers ______ move to the safe areas.
　　A. would B. could
　　C. were able to D. need
　　分析 C 盡管地震襲擊這個村莊,村上的人成功地撤到了安全地帶。were able to表示“成功做了某事”。
　　can和could表示有能力,但不一定做到。例如:
　　He could be in time for the first class this morning, but there was a traffic accident. 他本可準(zhǔn)時到校上第一節(jié)課,但路上發(fā)生了交通事故。
　　
　　can和may可用來表示“允許”或者征求對方的意見(請注意在疑問句和答句中的用法):
　　—Could I visit you again next Christmas?我能在明年圣誕節(jié)再來拜訪您嗎?(語氣委婉)
　　—Yes, you can. / No, I’m afraid not. 好啊。/ 恐怕不行。
　　—May I watch TV after finishing my homework? 做完作業(yè)可以看電視嗎?
　　—Yes, you may. / No, you mustn’t. / No, you’d better not. 可以。/ 不行!/ 最好別看。
　　will / would在疑問句中用于第二人稱時,表示詢問對方的意愿或向?qū)Ψ教岢稣埱?后者更為婉轉(zhuǎn):
　　Will / Would you get me some souvenirs when you visit Shanghai Expo?你可以在參觀上海世博會的時候給我?guī)┘o(jì)念品嗎?
　　在疑問句中,shall 用來征求對方的意見或者請求指示:
　　Shall I fetch some orange juice for you? 要不要我給你拿些橘子汁來?
　　Since everybody is here, shall we start the meeting now? 既然每個人都到了,我們開會吧?
　　There’s an applicant for the interview. Shall he come in right now? 有位來應(yīng)聘的。要不要他馬上進(jìn)來?
　　
　　must表示“必須,應(yīng)該”,是說話人的主觀意愿;否定式mustn’t表示“不應(yīng)該,不準(zhǔn),禁止”等:
　　We must observe the traffic rules. 我們必須遵守交通規(guī)則。
　　You mustn’t farm on the Internet as a student, which costs lots of time. 作為學(xué)生不能網(wǎng)上種菜,那很耗時間的。
　　have (has) to表示客觀上的“需要,不得不”,強(qiáng)調(diào)來自外界的義務(wù):
　　It is time for the PE lesson. I have to go now. 是體育課的時間了,我得走了。
　　He is old enough now. You don’t have to worry about him. 他年齡不小了,你不必為他擔(dān)心。
　　shall用于第二、三人稱,表示說話人的命令、警告、威脅、強(qiáng)制等意思:
　　If you didn’t do as I told you. You shall not watch TV this evening. 你沒按照我說的去做,所以你今晚不能看電視。
　　No one shall smoke in this area. 任何人都不可以在這個地方吸煙。
　　should 表示職責(zé)、義務(wù)、勸告等,主觀性強(qiáng):
　　You shouldn’t judge a man always by his appearance. 你不應(yīng)該總是以貌取人。
　　should和ought to的含義大致相同,ought to 強(qiáng)調(diào)義務(wù)或責(zé)任時,比should語氣強(qiáng):
　　You ought to go and see Mary tomorrow. 明天你應(yīng)該去看瑪麗。
　　will和would表示決心、意志、意愿等,用于各種人稱:
　　I will make the computer work even if I have to stay up all night. 哪怕我熬夜也要修好電腦。
　　She will help you if you can’t finish cleaning the classroom before 5:00. 如果你在5:00前不能完成打掃教室的任務(wù),她會幫助你的。
　　
　　表示推測的情態(tài)動詞有must, can, could, may, might, should等。must表示推測時,意思是“一定、準(zhǔn)會”,語氣肯定,有把握,只能用于肯定的猜測,可以推測現(xiàn)在正在發(fā)生的動作和過去發(fā)生的動作。can和could表示推測時,往往用于否定句或疑問句,can’t表示“一定不”。
　　may, might表示推測時,意思是“可能、也許”,語氣沒有must肯定。may, might表推測時,還可以用于否定句,意思是“可能不、也許不”,但不用于疑問句。might不表示過去時態(tài),只是語氣上比may更委婉,表示可能性更小。should表示推測時,其語氣比較肯定,并暗示其推測有一定的事實(shí)依據(jù)或者合乎客觀常理。例如:
　　This book should be found easily in the library. 在圖書館應(yīng)該很容易找到這本書的。(說話人認(rèn)為,圖書館應(yīng)該有很多這類書籍,所以應(yīng)該很容易找到。)
　　He must have enjoyed the film Avatar, otherwise, he wouldn’t want to see it a second time. 他一定很喜歡電影《阿凡達(dá)》,否則他就不會再想看第二遍了。
　　—Someone called you while you were away, but he didn’t say who he was. 你不在時有人打你電話了,但他沒說他是誰。
　　—Thanks. Who can it be?謝謝!他會是誰呢?
　　Look at that car! It must be driving at least 150 kilometers an hour. 看那車開得多快!起碼每小時150公里。(must be driving表示對現(xiàn)在正在發(fā)生的事情的肯定猜測)
　　There is no one in the classroom. All the students must have gone to the Charity Bazaar. 教室里沒有一個人,學(xué)生一定都去Charity Bazaar了。(must have gone表示對過去事情的肯定猜測) could have done / might have done“本來能夠做而實(shí)際上沒有做”,用來表示對過去沒有做成某事的遺憾或責(zé)備;should have done / ought to have done“本應(yīng)該做某事而實(shí)際上沒有做”,用來表示責(zé)備或后悔;shouldn’t have done / ought not to have done“本不應(yīng)該做某事但卻做了”,用來表示責(zé)備或后悔;needn’t have done“本來不必做卻做了某事”。例如:
　　You should / ought to have come to the meeting earlier. 你應(yīng)該早點(diǎn)來開會。
　　He shouldn’t / ought not to have treated his parents like that. 他不應(yīng)該那樣對待自己的父母。
　　As you worked late yesterday, you needn’t have come this morning. 因?yàn)槟阕蛲戆疽构ぷ?沒有必要今天上午來的。
　　—I stayed at a hotel while in London. 在倫敦期間我住在旅館。
　　—Oh, why not contact Grace? You could have stayed at her home. 哦,你為何不聯(lián)系格雷西?你本來能住她家的呀!
　　
　　should意為“竟然”,表示驚奇、遺憾:
　　I’m surprised that you should be late today. 我很驚訝你今天竟然會遲到。
　　need表示“需要,必須”,多用于疑問句和否定句中:
　　—Need we buy a laptop?我們有必要買筆記本電腦嗎?
　　—No, we needn’t. / Yes, we must. 沒有必要。/ 有必要。
　　dare表示“敢”,多用在疑問句和否定句中:
　　Dare you swim across the river?你敢游到河對面去嗎?
　　She dare not speak in public. 她不敢在公共場所說話。
　　need和dare作為行為動詞時,其變化與一般動詞相同。作實(shí)義動詞的dare在否定句中其后的to可以省略:
　　We need to think it over. 我們需要仔細(xì)思考。
　　She didn’t need to do such a part-time job to cover her tuition expense. 她沒必要打工去掙學(xué)費(fèi)。
　　—Who dares to go? 誰敢去?
　　—I don’t dare (to) ask her. 我不敢去問她。
　　
　　即學(xué)即用
　　
　　1. It has been announced that all the passengers ______ remain in their seats until the plane lands safe.
　　A. could B. will
　　C. need D. shall
　　2. —It’s nearly 7 o’clock. John ______ be back!
　　 —Yes. I dare say he ______ be doing his extra work in his office now.
　　A. can; must B. will; might
　　C. should; must D. must; may
　　3. He asked for a sick leave yesterday; otherwise he ______ to buy the Expo tickets for you.
　　A. will go B. went
　　C. would have gone D. had gone
　　4. —Could I have the name of the girl who wears long hair next door?
　　 —Ok, if you ______ know it, her name is Diana.
　　A. can B. must
　　C. may D. should
　　5. Considering his ability and experience, he _____ better. What a pity!
　　A. need have done
　　B. must have done
　　C. can have done