假設法句型

If + Present Tense ..., S + will/shall + V ...
結構︰If+現在式…，S+未來式…
說明︰if 子句用現在式，表示須具備的條件，主句用未來式表示可能的結果，整句含意為“如果…，那麼…”。will 用於所有的人稱，在 I 和 we 之後可用 shall 代替 will。若主句的主詞為 I 或 we 時，可以用 shall 代替 will。像 if necessary(如果需要)，if possible(如果可能)，if so(如果這樣)等固定片語實際上是省略了的 if 子句。
If I lose my job, I will/shall go abroad. 如果我失業了，我就出國。
If I have a headache, I will take an aspirin. 如果我頭痛，我就服一片阿斯匹林。
If it rains, we will stay at home. 如果下雨，我們將待在家裡。
If the weather clears, we’ll go for a walk. 如果天晴，我們就去散步。
If the weather doesn’t clear, we won’t go for a walk. 如果天不晴，我們就不去散步。
Inflation may be rising, if (it is) so, prices will go up.
通貨膨脹率可能上升。如果是這樣，物價就會上漲。
If possible, She wants to go with us. 可能的話，她想跟我們一起去。
Sterling may fall, if (this should be) so, interest rates will rise.
英國貨幣可能貶值，如果果真如此，利率就會上升。

If + Present Tense ..., S + Modal Verb ...
結構︰If + 現在式，主詞+情態助動詞
說明︰本句型實為句型之變體。之所以用情態助動詞，主要是因為說話者對可能的結果不是太肯定或者是想表達例如必要的意思。
If it is fine tomorrow, we can go out. 如果明天天氣好，我門可以出門。
If it is fine tomorrow, we could go out. 如果明天天氣好，我門能夠出門。
If it is fine tomorrow, we may go out. 如果明天天氣好，我門可能出門。
If it is fine tomorrow, we might go out. 如果明天天氣好，我門可能出門。
If it is fine tomorrow, we should go out. 如果明天天氣好，我門宜於出門。
If it is fine tomorrow, we ought to go out. 如果明天天氣好，我門宜於出門。
If it is fine tomorrow, we must go out. 如果明天天氣好，我門必須出門。

If I should ..., Imperative Sentence.
結構︰If I+should ...+祈使語氣
說明︰代替，增加了條件的不確定性。本句型常用在想禮貌地提出要求或建議，或者想圓通地告訴他人該做什麼事的場合。用 should 起始句子即可不用 if 而表示條件。這種用法比較正式，常用於諸如公函之中，而不用於日常會話。
If you should see him, please give him my regards. 如果你見到他，請代我向他致意。
Should you see him, please give him my regards. 如果你見到他，請代我向他致意。
If you should happen to see him, please give him my regards.
如果你萬一見到他，請代我向他致意。
Should you happen to see him, please give him my regards.
如果你萬一見到他，請代我向他致意。
If you should write to her, send her my love. 如果你給她寫信，請代我向她問候。
Should you not wish our agent to call, please let us know.
你若不希望我們的代理人去造訪，請告之。
Should you be interested in our offer, please contact us.
你若對我們的報價感興趣，請與我們聯繫。

If I should ..., I will ...
結構︰If…should…，…would(will)/should(shall)等+原形動詞
說明︰這是表示與未來事實相反的假設<句型>，if <子句>要用<助動詞> should，譯成“萬一”。should可置於<主詞>前，將 if 省略。<主要子句>的<助動詞>則視可能性做變化：若可能性低，則為過去式，若可能性高，則為現在式。should 可以提前，而將 if 省略。
If he should come, I would tell him the truth. 萬一他來，我會把真相告訴他。
If he should come, I will tell him the truth. 萬一他來，我一定會把真相告訴他。
Should the boy come again, I would throw him out. 要是那男孩在來，我就把他攆出去。
If it should rain, our plan would be spoiled. 萬一下雨，我們的計劃就泡湯了。
Should that be true, the contract would be canceled. 假如那是真的，合約就該取消。
Should another world war break out, what would become of human beings?
萬一另一次世界大戰爆發，人類將會發生什麼事?

If I were to ..., I ...
結構︰If…were to…，…would/should 等+原形動詞
說明︰這也是表示與未來狀況相反的假設<句型>，與“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“與真理相反”的<假設語氣>。
If the sun were to rise in the west, he would pass the exam.
要是太陽從西邊出來，他就會通過考試--他不可能通過考試。
If I were to be born again, I would like to be a bird. 我要是再活一次，我想當一隻鳥。
If the sun were to disappear, what would the earth be like?
萬一太陽消失了，地球會變成什麼樣子?
Were I to die tomorrow, what would my children do?
萬一明天我死了，我的孩子們將怎麼辦呢?

If I + V-ed, I would ...
結構︰If…+過去式動詞…，主詞+過去式助動詞(would/should 等)+原形動詞
說明︰這是與現在事實相反的假設句型。if <子句>中的<動詞>，不論<主詞>為第幾人稱，一律用 were，were 可置於主詞前面，而省略 if。過去式<助動詞>除用 should(用於第一人稱)、 would(用於第二、三人稱)外，還可用不分人稱的 could，might 和 ought to，它們與<動詞>原形結合起來構成<假設語氣>。
If I knew his address, I could write to him. 如果我知道他的住址，我會寫信給他。
If I were free now, I might call on him. 如果我有時間，我可能會去看他。
If you were in my shoes (=Were you in my shoes), what would you do?
如果你站在我的立場，你會怎麼做?
Were I rich enough, I would buy a sports car. 如果我有足夠的錢，我就買一輛賽車了。
How happy I would be if I were a few inches taller! 要是我再高幾寸，我會好高興。
If I had a car, I could take her for a drive. 如果我有車， 我就帶她去兜風了。
If I had binoculars, I could see that comet clearly.
如果我有望遠鏡，我就可以清楚地看見那顆彗星。

It is time that ...
結構︰It is(high)time+(that)+主詞+過去式動詞
說明︰此句型意為“已經到了…的時間了”。過去式<動詞>處有時亦使用“should+<動詞>原形”。that <子句>也能用 to <不定詞>來表示。
It is time he went to bed. 該是他睡覺的時間了。
It is time that you left for the duty. 該你當班了。
It is time that you reflected upon your mistakes. 該是你反省思過的時候了。
It is high time he should come to our house for dinner.已經到了他該上咱家吃晚飯的時候了。
It is time for somebody to teach you to behave yourself.
該是有人教你應對進退之禮儀的時候了。

If I had V-ed, I would have ...
結構︰If…had+過去分詞…，主詞+would/should 等+have+過去分詞
說明︰這是與過去事實相反的假設<句型>。if <子句>要用<過去完成式>，<主要子句>則要用過去式<助動詞>(would，should，ought to，might，could)+have+過去<分詞>。if <子句>中的 had可置於<主詞>前面，而省略 If。
If I had seen him, I would have told him the truth. 如果我當時看見他，我會告訴他實情。
I could have finished the task if I had had more time.
如果當時我能有多一點時間，我就能完成這項任務。
If we had known of your departure, we would have seen you off at the airport.
如果我們事先知道你要離開，我們會到機場送行。
I someone had been with him, he might not have been drowned.
如果有人在他旁邊，或許他就不會溺死。
I might have been incredulous had I not been accustomed to such responses.
若非我已經習慣了這樣的回答，我可能不敢輕易相信。
Had he not apologize to her, she would not have forgiven him.
她(那時)不會原諒他，若非他道了歉。
Had he not saved me, I might have been drowned. 我可能溺死了，若非他救了我。
Had I proposed to you, would you have married me?
我要是當年向你求婚，你會答應嫁給我嗎?
Had you not helped us, we should have canceled this transaction.
假如沒有你的幫助，這筆生意早就告吹了。