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 ……

　　结构︰If…were to…，…would/should 等+原形动词

　　说明︰这也是表示与未来状况相反的假设，与“if …… should， ……”大致相同，但“if …… were to”所表示的可能性更低，通常用以表示“与真理相反”的。

　　If the sun were to rise in the west， he would pass the exam.

　　要是太阳从西边出来，他就会通过考试——他不可能通过考试。

　　If I were to be born again， I would like to be a bird. 我要是再活一次，我想当一只鸟。

　　If the sun were to disappear， what would the earth be like？

　　万一太阳消失了，地球会变成什么样子？

　　Were I to die tomorrow， what would my children do？

　　万一明天我死了，我的孩子们将怎么办呢？

　　If I + V-ed， I would ……

　　结构︰If…+过去式动词…，主词+过去式助动词（would/should 等）+原形动词

　　说明︰这是与现在事实相反的假设句型。if 中的，不论为第几人称，一律用 were，were 可置于主词前面，而省略 if.过去式除用 should（用于第一人称） would（用于第二、三人称）外，还可用不分人称的 could，might 和 ought to，它们与原形结合起来构成。

　　If I knew his address， I could write to him. 如果我知道他的住址，我会写信给他。

　　If I were free now， I might call on him. 如果我有时间，我可能会去看他。

　　If you were in my shoes （=Were you in my shoes）， what would you do？

　　如果你站在我的立场，你会怎么做？

　　Were I rich enough， I would buy a sports car. 如果我有足够的钱，我就买一辆赛车了。

　　How happy I would be if I were a few inches taller！ 要是我再高几寸，我会好高兴。

　　If I had a car， I could take her for a drive. 如果我有车， 我就带她去兜风了。

　　If I had binoculars， I could see that comet clearly.

　　如果我有望远镜，我就可以清楚地看见那颗彗星。

　　It is time that ……

　　结构︰It is（high）time+（that）+主词+过去式动词

　　说明︰此句型意为“已经到了…的时间了”。过去式处有时亦使用“should+原形”。that 也能用 to 来表示。

　　It is time he went to bed. 该是他睡觉的时间了。

　　It is time that you left for the duty. 该你当班了。

　　It is time that you reflected upon your mistakes. 该是你反省思过的时候了。

　　It is high time he should come to our house for dinner.已经到了他该上咱家吃晚饭的时候了。

　　It is time for somebody to teach you to behave yourself.

　　该是有人教你应对进退之礼仪的时候了。