结构︰If I+should ……+祈使语气
说明︰代替，增加了条件的不确定性。本句型常用在想礼貌地提出要求或建议，或者想圆通地告诉他人该做什么事的场合。用 should 起始句子即可不用 if 而表示条件。这种用法比较正式，常用于诸如公函之中，而不用于日常会话。
If you should see him， please give him my regards. 如果你见到他，请代我向他致意。
Should you see him， please give him my regards. 如果你见到他，请代我向他致意。
If you should happen to see him， please give him my regards.
如果你万一见到他，请代我向他致意。
Should you happen to see him， please give him my regards.
如果你万一见到他，请代我向他致意。
If you should write to her， send her my love. 如果你给她写信，请代我向她问候。
Should you not wish our agent to call， please let us know.
你若不希望我们的代理人去造访，请告之。
Should you be interested in our offer， please contact us.
你若对我们的报价感兴趣，请与我们联系。
If I should ……， I will ……
结构︰If…should…，…would(will)/should(shall)等+原形动词
说明︰这是表示与未来事实相反的假设，if 要用 should，译成“万一”。should可置于前，将 if 省略。的则视可能性做变化：若可能性低，则为过去式，若可能性高，则为现在式。should 可以提前，而将 if 省略。
If he should come， I would tell him the truth. 万一他来，我会把真相告诉他。
If he should come， I will tell him the truth. 万一他来，我一定会把真相告诉他。
Should the boy come again， I would throw him out. 要是那男孩在来，我就把他撵出去。
If it should rain， our plan would be spoiled. 万一下雨，我们的计划就泡汤了。
Should that be true， the contract would be canceled. 假如那是真的，合约就该取消。
Should another world war break out， what would become of human beings?
万一另一次世界大战爆发，人类将会发生什么事?
If I were to ……， I ……
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