★ I couldn't have got to Slough in time unless I'd had a helicopter.

除非我有一架直升飞机，否则我是不可能在那个时候赶到S地的。

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一般的英语语法书和辞典都会谈到，连接词unless的涵义和if...not相同，它们可以互换使用。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). 其实，这样的提法是不够全面的，因为存在着这么一个问题：unless和 if...not是否在各种场合下都能互换使用呢？现以上例--I couldn't have got to Slough in time unless I'd had a helicopter. 为例来探讨一下这个问题.

此句引自国外报刊。说话人被控犯了谋杀罪，在法庭他自辩说："发生此案时，我并不在S地，而是在离S地几十里外的另一个小镇上，关于这点我可以提供人证......"接着他说道："I couldn't have got to Slough in time unless I'd had a helicopter." 如果遵照一般语法书所述的公式：unless=if...not，将此句改作I couldn't have got to Slough in time if I'd not had a helicopter那会产生怎样的后果呢？若改作if...not，其涵义是：I did have a helicopter and I did get to Slough in time to commit the murder. 显而易见，这并不是犯人所想说的话。因此 unless是在大多数场合下可与if...not互换使用，但在某些场合里(例如在某些谓语动词用虚拟语气的句子里)却不能相互置换，否则它们会产生截然相反的涵义。

值得注意的是，unless用在虚拟语气句中时，存在着两种可能性。在某些句子中，它表示if...not的涵义，可以和if...not互换使用。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.

要不是你们来帮忙，我是不可能完成任务的。

然而，在另一些场合里，unless并不是表示 if...not的涵义，而是表示近似于but...if的涵义。这时候，可以改写为but...if，但决不能用if...not去替代。例如，犯人在法庭上所说的那一句，可以改写为：I could not have got to Slough, but I could have got to Slough if I had had a helicopter.

还有一点应当注意的是，unless句型和if...not句型具有不同的语言色彩和表达风格，前者是积极的说法，态度明朗，语气有力；后者是退一步的说法，态度含糊，语气软弱无力。

现在，你能否说出下列两个句子的不同涵义？

I should not expect their help unless they were my friends.

I Should not expect their help if they were not my friends.
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