but用作关系代词的用法问题

问题描述

There is no tree but bears some fruit. 没有树不结果。
这是一句谚语，老师解释说句子中的 but 不是并列连词，而是关系代词，在此句中用作主语。但我从未见过 but 用作关系代词的说法。请专家老师解释一下，非常感谢！

老师答疑

黎老师：

《薄冰英语惯用法词典》指出：
but 用作关系代词，等于that (或 who, which)... not。主句中通常有否定词（如 no, none 等）或者疑问词。例如：
No one is in the world but knows the Great Wall. 世上无人不知长城。
but 在从句中作主语。but knows 相当于 who doesn't know 。这样，整个句子就是双重否定，语气强于一般肯定句，有强烈的感染力。又如：
There is no one of them but is ready to help the people in trouble. 他们个个都乐意帮助有困难的人。(but is ready 相当于 who is not ready)
Who but knows it? (=Who [is there] that does not know it?) 谁不知道呢？

曾老师：

同意黎老师的分析。

but用作关系代词引导定语从句必须符合两个条件：

1. 主句部分必须是否定式或带有否定意义的词（no, little, few, hardly, barely, seldom, nobody, nothing...）。

2. but作关系代词相当于which/that/who + 否定式的谓语

即：整个复合句是以双重否定的形式来加强语气。例如：

There is no mother but loves her own children．没有不爱自己孩子的母亲。(butloves = who/that does not love)

I find few but admire him for his talents. 我发现很少有人不钦佩他的才华。(but admire =that/who don't admire)

He hardly has problems but can be solved. 他简直就没有解决不了的问题。(but can be solved =which/that can't be solved)

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