If 所引導的假設語氣
If 所引導的假設語氣有下列幾種句型：
should
would
1. 表達跟未來事實相反：If+ S+ should+ 原形動詞，S+ could
were to might
+原形動詞
例：
1. If he should come, I would tell the truth.
2. If the sun were to rise in the west, I would marry you.
3. If photosynthesis(光合作用) were to stop, all living things would disappear.
註：If+ S+ should+原形動詞是表達“一般情況不太可能”，而If+ S+ were to+原形動詞是表達“絕對與事實真理相反”。
would
should
2. 表達跟現在事實相反：If+ S+ were ，S+ could +原形動詞
過去式 might
例：
1. If I were you, I might leave at once.
2. I would buy that house if I had enough money.
would
should
3. 表達與過去事實相反：If+ S+ had+ p.p., S+ could +have+ p.p.
Might
例：
1. If you had come to the party, you might have met Jane.
2. He might have come if it had not rained.
4. 關於假設語氣應注意事項：
(a) 要利用已知動詞去判斷未知動詞，請思考下列空格的動詞結構
If+ S+ were to+原形動詞，S+____.
S+ might+ have+ p.p.+ if+ ____.
(b) 假設語氣的助動詞一律用過去式，be動詞一律用were。
(c) 假設語氣If子句可以改為倒裝結構，即(1)去掉if，(2)將should, were或had移至主詞前面。
例：
1. If he should come, I would tell the truth.
= Should he come, I would tell the truth.
2. If the sun were to rise in the west, I would marry you.
= Were the sun to rise in the west, I would marry you.
3. If I were you, I might leave at once.
= Were I you, I might leave at once.
4. You might have met Jane if you had come to the party.
= You might have met Jane had you come to the party.
請務必要掌握倒裝結構，才能判斷是與未來、現在、或過去事實相反。
would
should
(d) 與過去事實相反的事若影響到現在，則主要子句動詞用 could +原形之
might
結構，這種情況主要子句會有現在的時間副詞出現。
例：
1. If they had built more houses ten years ago, the housing problem would not be so serious now.
