英语动词语气有三种：陈述语气，祈使语气和虚拟语气。语气表示说话人对劝词所示示的动作或所处的状态持有的态度或看法。
1.辨别if 引导真实条件句和if引导的虚拟条件句的区别。
If he has time, he will go with us.= Probably he has time and will go with us.
If he had time, he would go with us.=But in fact he has no time.
2.虚拟条件句中主句和从句的谓与动词构成形式如下表
3.混合时间的虚拟语气
如果条件句中的动作和主句的动作不是同时发生，主句和从句的谓语动词的形式应分别根据各自所表示的时间加以调整。
1)If I had received the passport yesterday, I would start today.
2)If he had telephoned me last night, I would see him now.
3)If he had followed the doctor’s advice, he would be all right now.
4)If China had not been liberated, the working people would still be leading a miserable life.
4.should/ could / might/ ought to + have done 表示“过去本应该/可以做而实际上却没做”
needn't have done 表示“过去没必要作而实际上做了”。
5.虚拟语气中的倒装句
如果虚拟语气的条件从句谓语动词中含有were, had, could, should，有时可将if省去，而将条件从句的主语置于were, had, should, could 之后。
Had you invited us, we would have come to your party.
Were I you, I would do more practice after class.
Could she lend us a helping hand, she would do so.
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