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表示与现在事实相反时的谓语形式从句
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主句
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过去式 (be和were)
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would (should、could, might)+ 动词原形
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If Tom were hungry, he would eat the bread. (In fact, Tom isn’t hungry.
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（表示与过去事实相反的谓语形式）从句
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主句
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had + 过去分词
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would (should、could, might) + have +过去分词
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例如：If I had left a little earlier, I would have caught the early bus. (In fact, I didn’t catch the bus.)
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表示与将来事实相反的谓语形式 从句
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主句
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动词过去式、should + 动词原形、were to + 动词原形
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would (should、could, might) + 动词原形
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If they came tomorrow, we would told them about it. (In fact, they won’t come tomorrow。)
should do 条件从句中不管什么人称都用should do，可表示有偶然实现的可能性。
对比: 将来不太可能实现的三种非真实条件句中的虚拟语气对比
If it snowed tomorrow, I would stay at home. 如果明天下雪的话，我就留在家里。（常用形式）
If it should snow tomorrow, I would stay at home. 如果明天下雪的话，我就留在家里。（可能性较小）
If it were to snow tomorrow, I would stay at home. 如果明天要下雪的话，我就留在家里。（可能性最小）
省略if的条件句如条件句中有were, had, should时可以省略if，而把were, had, should放在主语前（倒装）
Were it necessary, I might go without delay. 如果需要的话，我可以立即去。（= If it were necessary......）
Had you taken my advice, you wouldn't have failed in the exam. 你要是听了我的建议，就不会考试不及格
