2017大学英语六级基础语法知识：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.
