1. 表示一种与过去事实相反的假设情况，条件从句中的谓语动词要用“had+过去分词”，主句中的谓语动词要用“would / should / could / might+have+过去分词”。 如： If you had come here yesterday, you would have seen him. 如果你昨天来到这里，你就会看见他了。 2. 表示一种与目前事实相反的假设情况，条件状语从句中的谓语动词要用一般过去式(be动词常用were)，而主句中的谓语动词则要用“would / should / could / might+动词原形”。
如： If I had his telephone number, I would call him now. 如果我有他的电话号码，我现在就打电话给他。 If I were you, I would not tell him that. 如果我是你，我就不把那件事告诉他。
3. 表示与将来的事实可能相反的假设情况时，条件状语从句中的谓语动词要用：1) should+动词原形，或2) 一般过去式，或3) were+不定式，主句中的谓语动词要用“would+动词原形“。
如： If it snowed tomorrow, many crops would die of cold. = If it were to snow tomorrow, many crops would die of cold. = If it should snow tomorrow, many crops would die of cold. 如果明天下雪，许多庄稼都会冻
死。
4. 有时候，条件和结果不是指同一个时间，这时，应根据上面谈到的几种情况，使用相应的形式，这种虚拟语气形式被称为错综虚拟语气。
如： If you had listened to the teacher carefully yesterday, you would know how to do the exercise now. 如果你昨天认真听了老师的讲话，你现在就知道如何做这道题了。 5. 使用虚拟语气时，有时候可以省略引导条件状语从句的连词if。这时，从句部分要使用部分倒装，即将从句部分的助动词were / had / should等提到主语前面。如： Were I you, I would not tell him that. Should it snow tomorrow, many crops would die of cold.
Had you listened to the teacher carefully yesterday, you would know how to do the exercise now
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