英语陷阱(4)

I couldn't have got to Slough in time unless I'd had a helicopter.
除非我有一架直升飛機, 否則我是不可能在那個時候趕到S地的.
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 一般的英語語法書和辭典都會談到, 連接詞unless的涵義和if...not相同,
它們可以互換使用. 例如: I  shan't go unless he asks me.
等于I shan't go if he does not ask me. (Eckersley's A comprehensive English Grammar);
 You will fail unless you work harder. 等于You will fail if you do not work harder.
(A.S. Hornby's The Advanced Learner's Dictionary of Current English).
其實, 這樣的提法是不夠全面的, 因為存在著這么一個問題: unless和 if...not
是否在各種場合下都能互換使用呢? 現以上例--I couldn't have got to Slough
in time unless I'd had a helicopter. 為例來探討一下這個問題.

 此句引自國外報刊. 說話人被控犯了謀殺罪, 在法庭他自辯說:"發生此案時,
我并不在S地, 而是在離S地几十里外的另一個小鎮上, 關于這點我可以提供人証......".
接著他說道:"I couldn't have got to Slough in time unless I'd had a helicopter."
如果遵照一般語法書所述的公式: unless=if...not, 將此句改作I couldn't have got to
Slough in time if I'd not had a helicopter, 那會產生怎樣的后果呢? 若改作if...not,
其涵義是: I did have a helicopter and I did get to Slough in time to commit the murder.
 顯而易見, 這并不是衽所想說的話. 國此, unless是在大多數場合下可與if...not互換使用,
但在某些場合里(例如在某些謂語動詞用虛擬語氣的句子里)卻不能相互置換,否則它們會產生截然相反的涵義.

 值得注意的是, unless用在虛擬語氣句中時, 存在著兩種可能性. 在某些句子中,
它表示if...not的涵義, 可以和if...not互換使用. 例如:
   Unless you had lent me a hand, I should not have fulfilled my task.
   = If you had not lent me a hand, I should not have fulfilled my task.
   要不是你們來幫忙, 我是不可能完成任務的.

 然而, 在另一些場合里, unless并不是表示 if...not的涵義, 而是表示近似于but...if的涵義.
 這時候, 可以改寫為but...if, 但決不能用if...not去替代. 例如, 衽在法庭上所說的那一句, 可以改寫為:
   I could not have got to Slough, but I could have got to Slough if I had had a helicopter.

 還有一點應當注意的是, unless句型和if...not句型具有沒的倏色彩和表達風格,
前者是積極的說法, 態度明朗, 語氣有力; 后者是退一步的說法, 態度含糊, 語氣軟弱無力.
 
 現在, 你能否說出下列兩個句子的不同涵義?
 I should not expect their help unless they were my friends.
 I Should not expect their help if they were not my friends.