1、Unit 2 语法情态动词一 基本概念情态动词作为谓语的一部分,无人称和数的变化;不能单独使用,与谓语动词构成完整谓语,不能单独作谓语,但在省略回答时,可单独使用。二 基本用法(一) can 和 could1. 表示能力或客观可能性,还可以表示请求和允许。e.g. Can you finish this work tonight?(表示能力) Man cannot live without air. (表示客观可能性) Can I go now? Yes, you can. (表示请求或允许) could 也可表示请求,语气委婉。主要用于疑问句,不可用于肯定句,答语应用can (即:could
2、 不能用于现在时态的简略答语中)。e.g. Could I come to see you tomorrow? Yes, you can. (否定答语可以用:No,Im afraid not.)can表示能力时 = be able to do e.g. Ill not be able to come this afternoon.注:can 只能用于一般现在时和一般过去时(could);be able to do 可以用于任何时态中。2. 表示惊异、怀疑、不相信的态度。(主要用于否定句、疑问句或感叹句中)e.g. Can this be true? How can you be so care
3、less! His cannot be done by him.3. can (could)+have+done 的疑问或否定形式表示对过去发生的行为怀疑或不肯定。e.g. He cannot have been to that town.Can he have got the book?4. 归纳拓展 cannot but 不得不e.g. I cannot but admire his courage. 我不得不佩服他的勇气。 cannot (couldnt)help+doing 情不自禁e.g. When I heard what the little girl said I couldn
4、t help laughing. 当我听见小女孩所讲的,不禁大笑起来。 cannot.too.怎么.也不过分e.g. You cannot be too careful in the choice of your friends.你在选择朋友时,怎么小心也不过分。 表示经过努力才能完成之事不可用could, 而只能用 be able to。e.g. Because he worked hard, he was able to pass the exam.因为他努力了才会通过考试。(二) may 和might1. 表示许可。表示请求、允许时,might比may 的语气更委婉一些,否定回答时要用m
5、ustnt,表示“不可以、禁止、阻止”之意。e.g. You drive the car. Might I use your pen? No, you mustnt. 2. 表示推测、可能(疑问句不能用于此意)。e.g. He may be very busy now.3. 用于祈使句中表示祝愿。e.g. May you succeed!4. 表示请求或规劝。e.g. You might pay more attention to me.5. “may (might)+ have done”表示对过去发生的行为的推测。e.g. He may not have finished the work
6、.6. 归纳拓展 may not 表示一种礼貌的“不可”。e.g. You may not leave the thing half done. might 可表示忠告;责备。e.g. You have broken tow dishes. You might try to be more careful. “may well+do原形”表示理所当然。e.g. You may well say so.你当然可以这样讲。may as well+do原形+as+动词原形 “与其.不如; 最好.不要。”e.g. You might as well throw your money away as l
7、end it to him.你要借钱给他,不如把钱扔了。(三) can和may1. 表示可能时,may 仅用于肯定句;而can 可用于各种句式。e.g. You may be right.你可能是对的。(may用于肯定句)You can be right.你可能是对的。(can用于肯定句)Can you be right?你可能是对的吗?(can 用于疑问句)You cannot be right. 你不可能是对的。(can 用于否定句)2. 表示允许时,二者意义相同,只是may 比较正式,而can比较口语化。e.g. May I smoke here? 我可以在这儿吸烟吗?Can I use
8、 your pen? 我可以用你的钢笔吗?(四) must 和have to1. must “必须”,强调的是一种主观看法,也表示责任或任务; have to “必须”,强调的是客观需要,有时态的变化。 e.g. You must take your doctors advice. (主观看法) 你必须采纳医生的建议。 You have to take your doctors advice. (客观需要) 你必须采纳医生的建议。You must obey the rules.(责任或义务)注:回答must引出的问句时,如果是否定回答,不能用mustnt,而用 neednt 或dont hav
9、e to.e.g.Must we hand in our exercise books today?Yes, you must.( No,you dont have to.)2. “must be+表语”,表示推测。其否定或疑问形式用can代替must.e.g. This must be your pen. 3. “must+have done”,表示对过去发生的行为的推测,其否定或疑问形式用can代替must.e.g. He must have been to Shanghai.(五) dare 和need1. need “需要”。作情态动词时,仅用于否定句或疑问句中。e.g. You ne
10、ednt come so early. Need I finish the work today? Yes, you must.注:neednt have done “本来没有必要做某事而实际上却做了”e.g. You neednt have waited for me.2. dare 作情态动词时,主要用于疑问句、否定句和条件句中,一般不用于肯定句中。e.g. How dare you say Im unfair?He darent speak English before such a crowd, dare he?3. dare 和need 常作实义动词。有时态、人称和数的变化。在肯定句
11、中,dare后面常接带to的不定式。在否定句和疑问句中,dare后面可接带to或不带to的不定式。e.g. I dare to swim across this river.He does not dare (to) answer. Dont you dare (to) touch it?(六) shall 和should1. shall 用于第一人称表示征求对方的意见。e.g. What shall we do next?2. shall 用于第一、第三人称疑问句中,表示说话人征求对方的意见或向对方请示。e.g. Shall we begin our meeting?When shall w
12、e leave the hospital?3. shall 用于第二、第三人称,表示说话人给对方的命令、警告、允诺或威胁。e.g. You shall fail if you dont work harder.(警告)He shall have a look when I finish reading.(允诺)He shall be punished. (威胁)4. 表示推测或可能。e.g. They should finish the work by now.5. should 表示劝告、义务、建议、命令,其同义词是 ought to, 在疑问句中通常用should 代替ought to.e
13、.g. We should learn some English.Should I open the window?6. should 可以用在if引导的条件句中,表示一件事听起来可能性很小,但也不一定完全没有可能;相当于“万一”的意思,从句谓语由“should+do原形”构成,主句谓语却不一定用虚拟语气。e.g. Ask her to ring me up if you should see her.你万一见到她,让她给我打个电话。If you should change your mind, please let me know.万一你改变主意,请通知我。7.“why/how等疑问副词+s
14、hould”,表示说话人对某事不能理解,感到意外、赞叹、愤怒、惊讶等感情,意思为“竟然”。e.g. Why should you be so late today?你今天怎么来这么晚? Where is Betty living? How should I know? 我怎么会知道!I dont know why you should think that I did it.我真不知道你凭什么认为这件事是我干的。8. should +have done “本应该做而实际上没有做”(虚拟语气)e.g. You should have started earlier.(七) will 和would
15、1. 表示请求、建议等,would 比will语气委婉、客气。e.g. Would you pass me the salt?2. 表示意志、愿望和决心。e.g. I will never do that again. They asked us if we would do that again.3. 表示习惯动作。e.g. Fish will die without water.4. 表示推测(主要用于第二、第三人称) will be 表示对目前情况的推测。e.g. This will be the book you want.will have done 表示对已经完成的动作或事态的推测
16、。e.g. He will have arrived by now.5. would “过去常常”,表示过去反复发生的动作或某种倾向,比used to do 正式,但没有“现在已无此习惯”的含义。e.g. During the vacation, he would visit me every week.6. 表料想或猜测。e.g. It would be about ten when he left home.I thought he would have told you all about it.7. would 还可用于虚拟语气。e.g. If I were you, I would try it again.