结构︰If＋现在式…，S＋未来式… 说明︰if 子句用现在式，表示须具备的条件，主句用未来式表示可能的结果，整句含意为“如果…，那幺…”。will 用于所有的人称，在 I 和 we 之后可用 shall 代替 will。若主句的主词为 I 或 we 时，可以用 shall 代替 will。像 if necessary（如果需要），if possible（如果可能），if so（如果这样）等固定词组实际上是省略了的 if 子句。 If I lose my job, I will/shall go abroad. 如果我失业了，我就出国。 If I have a headache, I will take an aspirin. 如果我头痛，我就服一片阿斯匹林。 If it rains, we will stay at home. 如果下雨，我们将待在家里。 If the weather clears, we’ll go for a walk. 如果天晴，我们就去散步。 If the weather doesn’t clear, we won’t go for a walk. 如果天不晴，我们就不去散步。 Inflation may be rising, if (it is) so, prices will go up.通货膨胀率可能上升。如果是这样，物价就会上涨。 If possible, She wants to go with us. 可能的话，她想跟我们一起去。 Sterling may fall, if (this should be) so, interest rates will rise. 英国货币可能贬值，如果果真如此，利率就会上升。 来源：考试大 If Present Tense ..., S Modal Verb ...来源：考试大
结构︰If 现在式，主词＋情态助动词 说明︰本句型实为句型之变体。之所以用情态助动词，主要是因为说话者对可能的结果不是太肯定或者是想表达例如必要的意思。 If it is fine tomorrow, we can go out. 如果明天天气好，我门可以出门。 If it is fine tomorrow, we could go out. 如果明天天气好，我门能够出门。 If it is fine tomorrow, we may go out. 如果明天天气好，我门可能出门。 If it is fine tomorrow, we might go out. 如果明天天气好，我门可能出门。 If it is fine tomorrow, we should go out. 如果明天天气好，我门宜于出门。 If it is fine tomorrow, we ought to go out. 如果明天天气好，我门宜于出门。 If it is fine tomorrow, we must go out. 如果明天天气好，我门必须出门。 来源：考试大 If I should ..., Imperative Sentence.
结构︰If I＋should ...＋祈使语气来源：考试大 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什幺事的场合。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 ...来源：考试大
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<句型>，if <子句>要用<助动词> should，译成“万一”。should可置于<主词>前，将 if 省略。<主要子句>的<助动词>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，人类将会发生什幺事？
If I were to ..., I ... 来源：考试大 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<句型>，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<假设语气>。 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，地球会变成什幺样子？ Were I to die tomorrow, what would my children do? 万一明天我死了，我的孩子们将怎幺办呢？