一、虚拟语气的定义：在英语中，表示说话人所说的话不是事实，而是一种假设、愿望、怀疑或推测，要用虚拟语气。
二、虚拟语气用于条件状语从句中:
1. 表示与现在事实相反的假设, 条件状语从句中的谓语动词用“过去式(be 动词的过去式用were)”, 而主句中的谓语动词“would / should/ could / might + 动词原形”;
e.g. If I were a boy, I would join the army.
What would you do if you won the lottery?
If I had a lot of money now, I would travel around the world.
If I had a lot of money, I would go shopping.
2.与过去的事实相反,从句的谓语用had ＋ 过去分词, 构成 If I (we, you, he, they)＋had＋过去分词的形式；
主句的谓语用should(第一人称)/would (第二、三人称) / could / might have ＋动词过去分词。
e.g. If he had driven more carefully, he wouldn’t have had the car accident yesterday.
If I had learnt how to swim, I wouldn’t have been trapped in this island.
3. 与将来事实相反,从句的谓语用If I (we, you, he, they) ＋动词过去式或 were to do 或should do 形式, 主句谓语用should(第一人称) /would (第二、三人称) /could/ might ＋动词原形。
e.g. If you succeeded, everything would be all right.
If you should succeed, everything would be all right.
If you were to succeed, everything would be all right.
三、混合条件句：主句与从句的动作发生在不同的时间, 这时主从句谓语动词的虚拟语气形式因时间不同而不同, 这叫做混合条件句。
e.g. If you had asked him yesterday, you would know what to do now.
四、without和but for 构成虚拟，but for意为“要不是”, without意为“没有”。遵循if引导的条件状语从句虚拟语气中主句的时态使用规则。
e.g. Without sunlight, people’s life would be different from today.
But for your help, I wouldn’t have finished the work.
五.专题练习:
1.改错：
(1) I shouldn’t do that if I was you.
(2) If my father were here now, he will tell me what to do.
(3) If I knew her telephone number, I would have called her.
(4) If it had not rained yesterday, he might come.
(5) I don’t think that I shall fail. But if I fail, I would try again.
2. 语法填空：
(1) I was stuck in the heavy traffic, Otherwise I _______(come) here earlier.
(2) If there were no subjunctive mood, English ____________much easier. (be)
(3) But for the leadership of our Party and Government, we __________ these splendid results. (not achieve)
(4) If I ____(be) ten years younger, I would accept the offer and do the job.
(5) I don’t know the word. If I____________ (know) the meaning of it, I would not have to look it up.
(6) He telephoned to inform me of your birthday, or I _________(know) nothing about it.
(7) _______(be) there no modern telecommunications, we would have to wait for weeks to get news from around the world.
(8) If it were to rain tomorrow, the sports meeting_______ (put off)
(9) If the world-war____(burst)out now again, what__________ (happen) to our people on earth?
(10) Had it not been for the heavy fog, you_____(see) the beautiful tower in the distance easily last Tuesday.
(11) If they _____ (be) here, they __________ (take) your advice.
(12) If Tom ________ (be) more careful in the exam, he ____________ (pass) it already.
(13) If you _______ (get) up earlier, you __________ (catch) the first train.
(14) If I ________ (meet) you yesterday, we __________ (go) to the concert together.
(15) If he ________(get) up early, he __________(be) on time.
(16) If there _________ (be) no natural resource any more, we ___________ (make) use of nuclear power.
(17) What ___________ (happen) if you _________ (get up) too late tomorrow?
(18) If Robots _________ (control) the world some day, here _________ (be) no friendship.
(19) If the staff_______(be) more careful, the mall ____(be) on fire now.
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