2017大學英語六級基礎(chǔ)語法知識：虛擬語氣
1. 表示現(xiàn)在/過去/將來情況的虛擬條件句
虛擬條件句主要有三種結(jié)構(gòu)：
1) 表示與現(xiàn)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。
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(gòu)成
動詞wish后的賓語從句表示未實現(xiàn)的或不可能實現(xiàn)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(gòu)成形式。
1) 表示現(xiàn)在不可能實現(xiàn)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(xiàn)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(xiàn)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(fā)生的時間是不一致的.如從句是對過去情況的假設(shè)，而主句可能是對現(xiàn)在正在進行情況的假設(shè)，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(dǎo)的從句”構(gòu)成。該結(jié)構(gòu)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.
