四六級常考語法之虛擬語氣
　　1. 表示現在/過去/將來情況的虛擬條件句
　　虛擬條件句主要有三種結構：
　　1)　表示與現在情況相反：主句謂語用“would / could / might +動詞原形”，從句謂語用“動詞的一般過去時”（動詞be的過去式一律用were）。
　　If I were you, I would not accept his offer.
　　If I had time, I would certainly go to the cinema with you.
　　2) 表示與過去情況相反：主句謂語用“would / could / might + have +過去分詞”，從句謂語用過去完成時。
　　If I had got up a little earlier, I wouldn’t have missed the train.
　　If I had been more careful, I might have passed that exam.
　　3) 表示與將來情況相反：主句謂語用“would / could / might +動詞原形”，從句謂語用“were +動詞不定式”或“should +動詞原形”。
　　If I were to do the job, I would not be able to have enough time to study.
　　If it should rain tomorrow, I would not go out with you.
　　2. 虛擬條件句連接詞if的省略
　　如果虛擬條件句的從句中含有were, had, should, could等詞時，可以省略連接詞if，但這時必須把were, had, should, could等詞移到主語前面，形成倒裝。這種句型主要用于書面形式。采集者退散
　　Were I to do the job, I would finish doing it within two weeks.
　　Had it not been for his help, we couldn’t have arrived there on time.
　　Should it rain tomorrow, I would stay at home.
　　3. wish后賓語從句中的謂語構成
　　動詞wish后的賓語從句表示未實現的或不可能實現的愿望，其賓語從句中的謂語動詞要用虛擬語氣，有以下三種構成形式。
　　1) 表示現在不可能實現的愿望，賓語從句中的謂語動詞用一般過去時。（be的過去式為were）。
　　I wish I had enough money to buy a car.
　　I wish I were as young and energetic as you.
　　2) 表示過去未能實現的愿望，賓語從句中的謂語動詞用過去完成時（had +過去分詞）或“would / could + have +過去分詞 ”。
　　I wish I hadn’t made such a mistake.
　　I wish I could have done it better.
　　3) 表示將來不可能實現的愿望，賓語從句中的謂語動詞用“would / should (could, might) +動詞原形”。
　　I wish I would not get old.
　　I wish I could travel around the world one day.
　　4. 某些動詞后的賓語從句中謂語動詞要用虛擬語氣
　　在一些動詞后面的賓語從句中，謂語動詞要用虛擬語氣。這時，謂語動詞用動詞原形或should +動詞原形。這類動詞一般表示命令，要求，決定，建議，主張等概念，主要有：advise, ask, command, decide, demand, desire, direct, insist, move, order, propose, recommend, request, require, suggest,等。
　　The workers demanded that their wages (should) be raised by 10 per cent.
　　The teacher decided that you do the experiment first.
　　5. 某些名詞后的表語從句和同位語從句中謂語動詞要用虛擬語氣
　　在一些名詞后面的表語從句和同位語從句中，謂語動詞要用虛擬語氣。這時，謂語動詞用動詞原形或should +動詞原形。這類名詞一般表示命令，要求，決定，建議，主張等概念，主要有：advice, idea, instruction, motion, order, plan, proposal, recommendation, request, requirement, suggestion,等。
　　My suggestion is that we (should) send for a doctor immediately.
　　He gave the order that they (should) do the experiment again.
　　6. 錯綜時間條件句
　　條件從句表示的動作和主句表示的動作，有時發生的時間是不一致的.如從句是對過去情況的假設，而主句可能是對現在正在進行情況的假設。 這種句子稱為錯綜時間條件句.
　　If the weather had been more favourable, the crops would be growing still better.
　　7. 某些主語從句中的謂語動詞要用虛擬語氣
　　某些表示命令，要求，決定，建議，主張以及“重要性”和“緊迫性”等概念的主語從句中謂語動詞要用虛擬語氣。這時，謂語動詞用動詞原形或should +動詞原形。這類主語從句一般由 “It is (was) + 形容詞／過去分詞 + that引導的從句”構成。該結構中，常用的形容詞主要有：advisable, appropriate, desirable, essential, imperative, important, necessary, obligatory, proper, urgent, vital等；常用的過去分詞主要有：arranged, decided, demanded, desired, ordered, proposed, recommended, requested, suggested等。
　　It is necessary that you (should) stop smoking so heavily.
　　It is arranged that you (should) enter for this competition.
　　8. as if / though引起的從句
　　當as if / though引起的從句所表達的內容完全與實際情況相反或者純粹是一種假設時，通常要用虛擬語氣。如果從句表示與現在的事實相反，謂語動詞用一般過去時；如果從句表示與過去的事實相反，謂語動詞用過去完成時；如果從句表示與將來的事實相反，謂語動詞用would (might, could) + 動詞原形；
　　They talked as if they had been friends for years.
　　I remember the whole thing as if it happened yesterday.
　　It looks as if it might rain.
　　Note: 如果as if / though引起的從句所表達的內容被看作是事實或者有可能是真實的，則要用陳述語氣。
　　It seems as if it is going to rain.
　　The meat tastes as if it has already gone bad.
　　9. lest, for fear that和 in case引起的從句
　　當lest, for fear that和 in case表示“以免，以防，生怕，惟恐”等時，在它們引起的從句中，謂語動詞常用should + 動詞原形。
　　He ran away lest he should be seen.
　　He’s working hard for fear that he should fall behind.
　　He left early in case he should miss the last train.
　　10. If only引出的從句
　　If only引出的從句用以表達感嘆性的愿望，常譯為“要是…就好了”。If only從句經常省略結果主句，且主要用在虛擬語氣中，即從句的謂語動詞用一般過去時表示現在或將來不可能實現的愿望，或用過去完成時表示過去沒有實現的愿望。
　　If only the rain would stop.
　　If only I’d listened to my parents.
　　Note: if only引出的句子偶爾也可使用陳述語氣，但考生須注意的是，在各類測試中一般都以用虛擬語氣為正確答案。
　　11. would rather（that）引出的從句
　　would rather意為“寧愿”，接從句時常省略關系代詞that。would rather后的從句要用虛擬語氣，即從句謂語動詞用一般過去時表示現在或將來的愿望，用過去完成時表示與過去事實相反的愿望。
　　I’d rather you told me the truth.
　　I would rather you came tomorrow than today.
　　I’d rather you hadn’t told me about it.
　　12. It is (about/high) time (that) 句型
　　該句型表示“（早）該做…”，其后的定語從句中的謂語動詞要用虛擬語氣表示。在測試中從句的謂語動詞用一般過去時。
　　It is time that we went to bed.
　　It is high time that somebody taught you to behave yourself.
　　13. 表示猜測的幾種不同的方法及意義
　　一些情態動詞與動詞的完成式連用能夠表示對過去情況的猜測或者未實現的可能性。
　　1) could have + 過去分詞
　　A. 表示對過去可能發生的事情的推測，意為，“可能做了某事”。
　　He couldn’t have seen her yesterday.
　　They could have lost their way.
　　B. 表示某事在過去本有可能發生，但事實上并未發生，意為，“本可以做某事”。
　　We could have started a little earlier.
　　I could have killed her. It was a narrow escape.
　　2) may (might) have + 過去分詞
　　A. 表示對過去情況的推測，意為，“可能已做某事”。
　　He may have heard the news.
　　I might have come to a wrong conclusion.
　　B. 表示一種未實現的可能性，即本可以做某事，而實際上并沒有做，有時含有抱怨的口吻。
　　It was a narrow escape. You might have killed yourself.
　　A lot of men died who might have been saved.
　　3) must have + 過去分詞
　　表示邏輯上的必然性，即按照某些現象推斷過去肯定發生過的某事。
　　She must have made a big mistake.
　　The city at one time must have been prosperous, for it enjoyed a high level of civilization.
　　4) needn’t have + 過去分詞
　　表示過去不必做某事，但已經做了，即本可不必做某事。
　　You needn’t have woken me up. I don’t have to go to work today.
　　He needn’t have watered the flowers, for it is going to rain.
　　5) should / ought to have + 過去分詞
　　表示過去應該做某事而實際上沒有做。
　　You should / ought to have done what your parents told you. (But you failed to do it.)
　　You should / ought to have been more careful. (But you weren’t.)
　　6) shouldn’t / oughtn’t to have + 過去分詞
　　表示過去不應該做某事而實際上卻做了。
　　You shouldn’t / oughtn’t to have crossed the road when the lights were red. (But you did.)
　　They shouldn’t / oughtn’t to have left so soon. (But they did.)
　　7) would have + 過去分詞
　　表示對現在或將來某時之前業已完成的動作的推測。可譯為，“可能”、“也許”、“想必”。
　　He would have arrived by now.
　　She would have recovered by then.