There is no rule but has exceptions. 凡是规则, 都有例外.

　 如同汉语一样, 英语也经常运用"否定之否定"的修饰手段来表达肯定的涵义.

上面这个句子的意思相当于Every rule has exceptions.

上一句是通过"双重否定"(double negation)或"加重否定"(cumulative negation)的no和but的连用来加强语势; 后一句是正面的说法, 语气较弱. 在上一个句型中,but是关系代词, 表达that(or who)not的意味.

　 请看几个类例:

There is no one of us but wishes to remove all the illegal dangerous structures in Hong Kong.

我们大家都希望拆除香港所有的僭建危楼.

There is no one but knows it.  此事无人不知.

There has not been a scholar but was a man of industry.

凡是有学问的人都是勤奋的

There is no man but errs. 凡是人都会犯错误.

There no man but thanked him.  人人都感激他.

　 在下列句子中, but是从属连词, 不是关系代词:

It never rains but it pours. 不雨则已, 一雨倾盆.

Not a day but it rains.  没有一天不下雨.

Nothing is so hard but it becomes easy by a lot of practice.

无论怎样难的事, 通过大量的实践就不难了.

no man is so foolish but he may give a piece of good advice sometimes.

有时候愚蠢的人也会给人出个好主意.

　 下面的句子也是属于"否定之否定"句型, 它们也具有加强语势的修辞效果:

No language is more exempt from error and obscurity than the language of mathematics.

任何语言都比不上数学语言那么精确和明瞭.

He never goes to Shatin without dropping in at the Back Piaza.

他每次到沙田总要到图书广场去转一趟.

　 下面的一些英语谚语也是采用"双重否定"的结构来表达"强化肯定"的涵义:

Never sweet without sweat.  不吃苦, 何来甜.

Never smoke without fire.  无风不起浪.

Nothing is impossible to a willing heart. 诚心诚意, 无事不成.

　 请注意, 在下列句中, but既不是关系代词, 也不是从属连词或并列连词, 而是介词:

No one but you can do it. 这件事只有你才干得了.

There is no hiding of evil but not to do it. 若要人不知, 除非己莫为.

　 在下列句中, 第一个but是动词, 第二个but是名词.

But me no buts . 请不要"但是,但是"地推托吧