核心问题
《新英汉辞典》(p.156)将 finish the task by the end of the month 译为“在月底前完成这任务”，从译文看，上面的句子 = finish the task before the end of the month。难道 by 与 before 就一点儿区别也没有了吗？
词典解答
before 与 by 在表示时间方面是有区别的, before 的含义是: earlier than, 而 by 的含义是: not later than, when the time indicated comes, at or before, 而且 by 强调时间的终点 (an end point)。因此, 汉译“……之前”没有体现出英语介词 by 与 before 的区别, 但有时又找不到更合适的译法。不过, 我们既然掌握了这两个词的区别, 我们在很多情况下还是有办法在译文里把它们区分开来的。
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He ought to be here by this time. 这个时候他应该来了。
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By the time we'd walked five miles, he was already exhausted. 当我们走了五英里的时候，他就已经筋疲力尽了。
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By the middle 1960s, the national networks were broadcasting most of their programs in color. 到了60年代中期全国电视网已经在播送大部分彩色电视节目了。