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四六级常考语法之虚拟语气
1. 表示现在/过去/将来情况的虚拟条件句
虚拟条件句主要有三种结构：
1) 表示与现在情况相反：主句谓语用“would / could / might +动词原形”，从句谓语用“动词的一般过去时”(动词be的过去式一律用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) 表示与过去情况相反：主句谓语用“would / could / might + have +过去分词”，从句谓语用过去完成时。
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) 表示与将来情况相反：主句谓语用“would / could / might +动词原形”，从句谓语用“were +动词不定式”或“should +动词原形”。
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. 虚拟条件句连接词if的省略
如果虚拟条件句的从句中含有were, had, should, could等词时，可以省略连接词if，但这时必须把were, had, should, could等词移到主语前面，形成倒装。这种句型主要用于书面形式。
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后宾语从句中的谓语构成
动词wish后的宾语从句表示未实现的或不可能实现的愿望，其宾语从句中的谓语动词要用虚拟语气，有以下三种构成形式。
1) 表示现在不可能实现的愿望，宾语从句中的谓语动词用一般过去时。(be的过去式为were)。
I wish I had enough money to buy a car.
I wish I were as young and energetic as you.
2) 表示过去未能实现的愿望，宾语从句中的谓语动词用过去完成时(had +过去分词)或“would / could + have +过去分词 ”。
I wish I hadn’t made such a mistake.
I wish I could have done it better.
3) 表示将来不可能实现的愿望，宾语从句中的谓语动词用“would / should (could, might) +动词原形”。
I wish I would not get old.
I wish I could travel around the world one day.
4. 某些动词后的宾语从句中谓语动词要用虚拟语气
在一些动词后面的宾语从句中，谓语动词要用虚拟语气。这时，谓语动词用动词原形或should +动词原形。这类动词一般表示命令，要求，决定，建议，主张等概念，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. 某些名词后的表语从句和同位语从句中谓语动词要用虚拟语气
在一些名词后面的表语从句和同位语从句中，谓语动词要用虚拟语气。这时，谓语动词用动词原形或should +动词原形。这类名词一般表示命令，要求，决定，建议，主张等概念，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. 错综时间条件句
条件从句表示的动作和主句表示的动作，有时发生的时间是不一致的.如从句是对过去情况的假设，而主句可能是对现在正在进行情况的假设。 这种句子称为错综时间条件句.
If the weather had been more favourable, the crops would be growing still better.
7. 某些主语从句中的谓语动词要用虚拟语气
某些表示命令，要求，决定，建议，主张以及“重要性”和“紧迫性”等概念的主语从句中谓语动词要用虚拟语气。这时，谓语动词用动词原形或should +动词原形。这类主语从句一般由 “It is (was) + 形容词/过去分词 + that引导的从句”构成。该结构中，常用的形容词主要有：advisable, appropriate, desirable, essential, imperative, important, necessary, obligatory, proper, urgent, vital等;常用的过去分词主要有：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引起的从句
当as if / though引起的从句所表达的内容完全与实际情况相反或者纯粹是一种假设时，通常要用虚拟语气。如果从句表示与现在的事实相反，谓语动词用一般过去时;如果从句表示与过去的事实相反，谓语动词用过去完成时;如果从句表示与将来的事实相反，谓语动词用would (might, could) + 动词原形;
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引起的从句所表达的内容被看作是事实或者有可能是真实的，则要用陈述语气。
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引起的从句
当lest, for fear that和 in case表示“以免，以防，生怕，惟恐”等时，在它们引起的从句中，谓语动词常用should + 动词原形。
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引出的从句
If only引出的从句用以表达感叹性的愿望，常译为“要是…就好了”。If only从句经常省略结果主句，且主要用在虚拟语气中，即从句的谓语动词用一般过去时表示现在或将来不可能实现的愿望，或用过去完成时表示过去没有实现的愿望。
If only the rain would stop.
If only I’d listened to my parents.
Note: if only引出的句子偶尔也可使用陈述语气，但考生须注意的是，在各类测试中一般都以用虚拟语气为正确答案。
11. would rather(that)引出的从句
would rather意为“宁愿”，接从句时常省略关系代词that。would rather后的从句要用虚拟语气，即从句谓语动词用一般过去时表示现在或将来的愿望，用过去完成时表示与过去事实相反的愿望。
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
该句型表示“(早)该做…”，其后的定语从句中的谓语动词要用虚拟语气表示。在测试中从句的谓语动词用一般过去时。
It is time that we went to bed.
It is high time that somebody taught you to behave yourself.
13. 表示猜测的几种不同的方法及意义
一些情态动词与动词的完成式连用能够表示对过去情况的猜测或者未实现的可能性。
1) could have + 过去分词
A. 表示对过去可能发生的事情的推测，意为，“可能做了某事”。
He couldn’t have seen her yesterday.
They could have lost their way.
B. 表示某事在过去本有可能发生，但事实上并未发生，意为，“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 + 过去分词
A. 表示对过去情况的推测，意为，“可能已做某事”。
He may have heard the news.
I might have come to a wrong conclusion.
B. 表示一种未实现的可能性，即本可以做某事，而实际上并没有做，有时含有抱怨的口吻。
It was a narrow escape. You might have killed yourself.
A lot of men died who might have been saved.
3) must have + 过去分词
表示逻辑上的必然性，即按照某些现象推断过去肯定发生过的某事。
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 + 过去分词
表示过去不必做某事，但已经做了，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 + 过去分词
表示过去应该做某事而实际上没有做。
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 + 过去分词
表示过去不应该做某事而实际上却做了。
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 + 过去分词
表示对现在或将来某时之前业已完成的动作的推测。可译为，“可能”、“也许”、“想必”。
He would have arrived by now.
She would have recovered by then.