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：
1. 表達跟未來事實相反：
should |
+ 原形動詞, |
S + |
should |
|
If +S + |
were to |
would |
||
+ 原形動詞 |
could |
|||
might |
||||
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”。
2. 表達跟現在事實相反：
S + |
should |
|||
If +S + |
were, |
would |
+ 原形動詞 |
|
過去式, |
could |
|||
might |
例：1. If I were you, I might leave at once.
2. I would buy that house if I had enough money.
3. 表達與過去事實相反：
S + |
should |
|||
If +S + |
had +p.p., |
would |
+ have + p.p. |
|
could |
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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。
(d) 與過去事實相反的事若影響到現在，則主要子句動詞用would/should/could/might +原形之
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.
其它假設語氣句型
除If之外，其它wish, as if, without等亦可引導假設語氣的結構。
1. wish所引導的假設語氣：
(a) 表達與未來事實相反：S+ with+ (that)+ S+ would/should/could/might+原形動詞
例：I wish I could go to the party tomorrow.
(b) 表達與現在事實相反：S+ wish+ (that)+ S+were/過去式
例：1. I wish I were a millionaire.
2. I wish I had enough money.
(c) 表達與過去事實相反：S+ wish+ (that)+ S+ had+ p.p.
例：I wish I had known he was in hospital.
註：上列結構that可以省略；此外， 假設語氣的be動詞一律用were。
2. as if (= as though, 似乎，好像)：as if所引導的假設語氣結構與上述wish的結構一樣。
(a) 表達與未來事實相反：S+ V+ as if+ S+ would/should/could/might +原形
例：He looks as if he would do it.
(b) 表達與現在事實相反：S+ V+ as if+ S+ were/過去式
例：1. He looks as if he were a rich man.
2. He looks as if he liked it.
(c) 表達與過去事實相反：S+ V+ as if+ S+ had+ p.p.
例：He talked as if he had read that book.
3. with表示“若有”，without表示“若無”，亦是引導假設語氣的結構。
(a) 表達與未來或與現在事實相反：With/Without+ NP, S+ would/should/could/might +have+ p.p.
例：With your help, I might succeed now.
(b) 表達與過去事實相反：With/Without+ NP, S+would/should/ could/might +have+ p.p.
例：Without your help, I could have failed then.
註：事實上，with+ NP與without+ NP語意上等於第一節If子句的部份，故主要子句所用的動詞是一樣的。
4. 表達命令要求、堅持、建議勸告等動詞所引導的假設語氣：這類動詞常考者包括order, command, demand, ask, require(命令要求)；insist, maintain(堅持)；suggest, propose, recommend, advise(建議勸告)。
(a) S+ V+ (that)+ S+ (should)+原形動詞：that與should皆可省略，若省略should，則後面動詞一律用原形。
例：1. I insisted (that) he (should) come back early.
2. My sister demanded (that) he (should) apologize (道歉)。
3. Jane suggested that I (should) buy a car.
(b) It be+ p.p.+ that+ S+ (should)+原形動詞
例：1. It was required that he (should) finish his work on time.
2. It is recommended that we (should) do more exercise.
(c) It be+ adj+ that+ S+ (should)+原形：這種結構的形容詞(adj)亦是表達命令要求、堅持、建議勸告的語氣，常考的包括necessary, important, imperative, essential, insistent, advisable等。
例：1. It is necessary that he (should) make an immediate decision.
2. It is advisable that we (should) do exercise at least twice a week.
5. It+ be (about或high) +time(that)+S+過去式：表達“該是‥的時候了”。
例：1. It is time that you worked hard.
2. It is about time you went to bed.
