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假設語氣
1. If + S + V…..，S + will/shall +原形動詞….
這種句型表示”如果….就…..”。 這是純條件假設句型，即這種條件是一種可能的事實，可以實現也可以不實現。 If子句要用現在式動詞，主要子句則要用現在式助動詞( will, shall, may, can, must, ought to , should) 加原形動詞。Should 等於 ought to，表示”應當”之一，是可以唯一在純條件假設中使用的過去式助動詞；could , might, would 就不可使用。If亦可被 provided (that)，
in case，suppose等取代； unless也可當做純條件假設語氣使用。
(1) If it rains, I shall stay at home.
如果下雨，我會待在家裡。
(2) If you see him, you should tell him the truth.
如果你看見他，就該告訴他事實。
(3) Provided (that) you help us move the box, we will be very grateful.
如果你肯幫我們搬這箱子，我們將感激不盡。
(4) You will miss the train unless you start right now.
除非你現在出發，否則你會錯過火車。
2. If…should…+ 祈始句
增加了條件的不確定性；其後的祈始句常用來禮貌性地提出要求或建議，或者圓通地告訴他人該做什麼事的場合。 此句型還可用should起始句子，即可不用if而表示條件。這種用法比較正式，常用於諸公函之中，而不用於日常會話。
(1) If you should see Mary, please give her my regards.
如果你見到瑪莉，請代我向他致意。
(2) Should you have any difficulty in the matter, please let us know.
如果你有任何困難，請告知。
3. What if / Say….
此句型意為”假使…..； 假若…..”。 What if 和 Say 用來引導一個假設條件句，其義相當於 Let us suppose(讓我們假設)。
(1) What if it rains when we can’t get under shelter?
若下起雨來，我們又沒處 避雨可怎麼辦?
(2) Say you were chosen to go there? What would you do ?
假設你被選上去那兒了呢?你將怎麼辦?
4. …as if / as though….
as if, as though 是副詞連接詞， 引導副詞子句， 表示”彷彿…..；好像……一樣”。子句中，可用現在式或未來式，已表示事實，常見的有 It seems as if , It looks as if等；也可用過去式或過去完成式，分別表示與現在或過去事實相反的假設語氣。
(1) He walks as if he is drunk.
他走起路來好像他真的醉了。
(2) It looks as if it is going to rain.
好像要下雨了。
5. If ….should…., S + 助動詞 + 原形動詞….
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，議成”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，則為未來式。
(1) If he should come, I would tell him the truth.
萬一他來，我會把真相告訴他。
(2) If he should come, I will tell him the tell him the truth.
萬一他來，我一定會把真相告訴他。
(3) Should the boy come again, I would throw him out.
要是那男孩再來，我就把他攆出去。
6. If I were …., I might…..
這是與現在或過去事實相反的假設句型。表示與現在事實相反時，if 子句中的be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 ，後接原型動詞。表示與過去事實相反時，主要子句用”助動詞+have+過去分詞”。
(1) If I were free now, I might call on him.
如果我有時間，我可能會去看他。
(2) 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。
(3) If I had seen him, I would have told him the truth.
如果我當時看見他，我會告訴她實情。
7. If…were to …., S would….
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。
(1) 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?
(2) 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?
8. If it were not for…., S would….
“If it were not for…”表示”若非(有)….”，用於與現在事實相反的假設語氣。意可用”but for +名詞”或 “without+名詞”代替。 Were可以提前，省略 if，如例句3。與過去事實相反的假設語氣用” If it had not been for…”。
(1) If it werenot for his presence, I would punish you.
要不是他在場，我會處罰你的。
(2) But for your advice, I should have failed.
要不是你的忠告，我會失敗的。
(3) I should be lonely, were it not for the birds and the flowers.
我會寂寞，若非有鳥也有花。
(4) If it had not been for his help, I should have failed.
如果沒有他的幫忙，我應該已經失敗了。
9. If I had + 過去分詞….，I could ….
這是時態不一致的假設句型，亦稱混合假設條件句。即: if子句若與過去事實相反，用過去完成表示；而主要子句若與現在事實相反，則用過去式助動詞表示。此類主要子句句尾通常有表現在的時間副詞，如 now, today 等。
(1) If I had read the text carefully, I could give the right answer today.
如果我認真閱讀了文章，今天我就能給出正確答案了。
(2) If you had studied hard, you might become someone.
如果你當初學習努力的話，你或許已經成了重要人物了。
(3) Had she followed her mother’s advice, she might be a pianist now.
當時他要是聽了媽媽的建議，線再她可能就是一位鋼琴家了。
10. If only + 過去式/ 過去完成式
本句型表示” 要是…就好了”。用過去式，表示與現在事實相反；若用過去完成式，則表示與過去事實相反。
(1) If only he had been here.
要是當時他在這就好了。
(2) If only I had another chance.
要是我再有一次機會就好了。
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