情態(tài)動詞有can(能)，may(可以)，must(必須)，have to(不得不)，ought to(應(yīng)該)，dare(敢)，used to(過去經(jīng)常)，had better(最好)，would rather(寧愿)。在肯定句中它們后邊都要接動詞原形。在否定句中，can，may，must和dare后邊加not;have to和ought to分別在have和ought后加not;used to的否定式可用used not to，也可用didn't use to，但后者用得較多;had better和would rather的否定式分別是had better not和would rather not。這些情態(tài)動詞與現(xiàn)在完成時連用在某種意義上有一定的虛擬性。

　　肯定句和否定句中的情態(tài)動詞

　　一、情態(tài)動詞的現(xiàn)在式在肯定句中的比較

　　1. can表示體力或腦力方面的“能力”、“技能”或根據(jù)客觀條件能做某種動作的“可能性”，但表示人體力或智力的具體動作時須用be able to。

　　1)Mild forms of execrise can some of the loss of flexibility that accompanies aging.

　　[A] stop[B] to stop[C] stopping[D] be stopped

　　He was able to do that without any help.他不需要任何幫助就能完成這項工作。

　　2. may表示“允許，可以”，相當(dāng)于be allowed to

　　2)If there is social or political change in a region [A] where a standard language [B] is spoken，local varieties [C] of the language may developing [D] .

　　may或might可和as well連用，表示“建議”，譯為“還是……的為好”

　　You may as well keep a certain distance from that mad man. 你們還是離那瘋子遠點為好。

　　You might as well go home now.你還是現(xiàn)在回家為好。

　　3. must表示“必須”或“應(yīng)當(dāng)”、“一定”

　　3)The formation of snow must be occurring [A] slowly [B] ，in calm air，and at a temperature near [C] the freezing point [D] .

　　4. have to 加動詞原形，表示“不得不”，“必須”，它比must更強調(diào)客觀

　　Tom had to work into the deep night everyday to earn a living. 湯姆為了生計每天都得工作到深夜。

　　5. should 表示“勸告”，“建議”或“義務(wù)”時，譯作“應(yīng)當(dāng)”，或表示“預(yù)測”和“可能”

　　He should take care of his parents as they are old enough not to live on themselves. 由于父母親老了，不能自理，他應(yīng)當(dāng)照顧他們。

　　He should be there now. 他可能到了。

　　should have done在虛擬語氣中表示“責(zé)備或后悔”

　　6. ought ，只有一種形式，即ought后必須加to，然后跟接動詞原形表示“有義務(wù)”或“必要”做某事，譯為“應(yīng)當(dāng)，應(yīng)該”

　　4)The traditional goal of science is to discover how things are，not how they ought .

　　[A] to[B] to be[C] be[D] have been

　　5)You are quite right;I am inferring in my comments [A] that McGraw had not ought to [B] have broken [C] in the room without his permission [D] .

　　7. dare 可以用作情態(tài)動詞，后面跟不帶to的動詞不定式，這主要用于否定句中，它本身可有現(xiàn)在時第三人稱單數(shù)，詞尾加s，它還可以有ing分詞形式(daring)和過去式及ed分詞形式(dared)

　　6)Although Oriental ideas of woman's subordination to man prevailed in those days，she meet with men on an equal basis.

　　[A] did not dared [B] dared not [C] dared not to [D] did dare not to

　　二、情態(tài)動詞在一般時否定句中的用法

　　can't(can not，cannot) 表示“不可能”，may not 表示“不可以”，mustn't(must not)表示“一定不要”，“不許可”，needn't (need not)表示“不必”，dare not +動詞原形 表示“不敢”

　　He can't finish his essay by this time. 現(xiàn)在他不可能寫完論文。

　　He may not sleep now. 他或許現(xiàn)在沒在睡覺。

　　You mustn't criticize her in that way. 你不應(yīng)那樣批評她。

　　You needn't come tomorrow. 你明天沒必要來了。

　　He dared not meet his fiance. 他不敢見女朋友。