情态动词有can(能)，may(可以)，must(必须)，have to(不得不)，ought to(应该)，dare(敢)，used to(过去经常)，had better(最好)，would rather(宁愿)。在肯定句中它们后边都要接动词原形。在否定句中，can，may，must和dare后边加not;have to和ought to分别在have和ought后加not;used to的否定式可用used not to，也可用didn't use to，但后者用得较多;had better和would rather的否定式分别是had better not和would rather not。这些情态动词与现在完成时连用在某种意义上有一定的虚拟性。

　　肯定句和否定句中的情态动词

　　一、情态动词的现在式在肯定句中的比较

　　1. can表示体力或脑力方面的“能力”、“技能”或根据客观条件能做某种动作的“可能性”，但表示人体力或智力的具体动作时须用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.他不需要任何帮助就能完成这项工作。

　　2. may表示“允许，可以”，相当于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连用，表示“建议”，译为“还是……的为好”

　　You may as well keep a certain distance from that mad man. 你们还是离那疯子远点为好。

　　You might as well go home now.你还是现在回家为好。

　　3. must表示“必须”或“应当”、“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 加动词原形，表示“不得不”，“必须”，它比must更强调客观

　　Tom had to work into the deep night everyday to earn a living. 汤姆为了生计每天都得工作到深夜。

　　5. should 表示“劝告”，“建议”或“义务”时，译作“应当”，或表示“预测”和“可能”

　　He should take care of his parents as they are old enough not to live on themselves. 由于父母亲老了，不能自理，他应当照顾他们。

　　He should be there now. 他可能到了。

　　should have done在虚拟语气中表示“责备或后悔”

　　6. ought ，只有一种形式，即ought后必须加to，然后跟接动词原形表示“有义务”或“必要”做某事，译为“应当，应该”

　　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 可以用作情态动词，后面跟不带to的动词不定式，这主要用于否定句中，它本身可有现在时第三人称单数，词尾加s，它还可以有ing分词形式(daring)和过去式及ed分词形式(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、情态动词在一般时否定句中的用法

　　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”，“不许可”，needn't (need not)表示“不必”，dare not +动词原形 表示“不敢”

　　He can't finish his essay by this time. 现在他不可能写完论文。

　　He may not sleep now. 他或许现在没在睡觉。

　　You mustn't criticize her in that way. 你不应那样批评她。

　　You needn't come tomorrow. 你明天没必要来了。

　　He dared not meet his fiance. 他不敢见女朋友。