在虚拟语气中，如果条件句中有had、were或should，我们就可以把had、were或should提前到句首，省略if，形成倒装句式。这三个词分别是三个时态下的虚拟。
一、had用于过去时的虚拟
1、If I had read that book, I would have told you.
Had I read that book, I would have told you.要是我读过那本书，我就告诉你了。
2、If you had arrived ten minutes earlier, you would have seen the star.
Had you arrived ten minutes earlier, you would have seen the star.要是你早到十分钟，你就能看见这个明星了。
二、were用于现在时的虚拟
1、If I were you, I would take his offer.
Were I you, I would take his offer.如果我是你，我就会接受他的出价。
2、If it were not for your advice, I coundn’t win the match.
Were it not for your advice, I coundn’t win the match.要不是你的建议，我不会赢得这个比赛。
三、should用于将来时的虚拟
1、If it should be sunny this weekend, we would have a campfire party.
Should it be sunny this weekend, we would have a campfire party.如果这个周末晴天，我们就举办一个篝火晚会。
2、If Tom shouldn’t arrive on time, we would have to turn to John instead.
Should Tom not arrive on time, we would have to turn to John instead.如果汤姆没有按时到达，我们就只能找约翰了。
注意，在否定句中，我们只能提前had、were或should，不能提前not，not还是放在原来位置。
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