1.表示与现在事实相反的假设时
从句谓语 主句谓语
过去时（be的过去时用were）would（should,could,might）+动词原形
例如：If I were you,I would choose to leave.
If Bill had time,he could attend the meeting.
2.表示与过去事实相反的假设时
从句谓语 主句谓语
had+过去分词 would（should,could,might）+have+过去分词
例如：If you had followed your mother’s advice,you would have got that company.
3.表示对将来发生情况的怀疑或实现的可能性很小时
从句谓语 主句谓语
过去时或should（were to）+动词原形should（would,might,could）+动词原形
例如：If you came tomorrow,we would have the meeting.
If the sun were to rise in the west,I would believe your story.
If it should rain tomorrow,I would stay at home.
以上句型可以转换成下列形式：
省略if，将were，had，should放在主语前面。例如：
Were I in school again,I would work harder.
Had I arrived earlier,I would have met her.
注意：在非真实条件状语从句中，省略连词的倒装形式的句首不能用动词的缩略形式。如：Were it not for the expense,I would go with you.
不能说：Weren’t it for the expense,I would go with you.
有时假设情况并不以条件从句表现出来，而是通过介词短语来表示，或通过上下文或其他方式表示出来。例如：
But for your help,I would fail the exam.
Without air,we would not be alive.
Suppose/Supposing it rained,what should we do?
4.错综时间条件句
如果主句与从句的动作发生的时间不一致，谓语动词形式应当作相应的调整。例如：
If the enemy hadn’t been driven away,we would still be suffering.
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