虚拟语气

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与现在事实相反：
if 主语+动词一般过去式(如果是系动词全用were) , 主语 + would(should,might,could,ought to) + 动词原形

if i were rich,i would help you
if you had money,you should help the poor

与过去事实相反：

if 主语+ had + 过去分词，主语 + would(should,might,could,should,ought to) + have + 过去分词

if i had arrived earlier, i could have met him

与将来事实相反:

低可能性：

if + 主语 + should + 动词原形，主语 + would(should,might,could,ought to) + 动词原形
if it should rain,i would stay home

高可能性:

if + 主语 + should + 动词原形，主语 + will(can,may,should ought to) + 动词原形

if it should rain,i will stay home