核心问题
but 与 however 都作“但是”“然而”解，而且都可以引出并列分句。请说明它们之间的区别。
词典解答
从语义上看, but 所表示的是非常明显的对比, 转折的意味较 however 为强。从语法上看, but 是个并列连词, 而 however 却是个联加副词 (conjunct)。从语序上看, but 总是位于它所引出的分句之首, 而 however 却可位于分句之首、之中或之尾, 但是译成汉语时一定要把它放在分句之首:
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You can phone the doctor if you like. However, I very much doubt whether you will get him to come out on a Saturday night. (CGEL) 如果你愿意你可以给医生打电话。但是，我非常怀疑你是否能在星期六的晚上把他请出来。
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On the question of style, however, I will make no concession. (Yeats) 但是, 关于风格问题, 我决不会让步。
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He said that it was so, he was mistaken, however. (Palmer) 他说事情是这样,但是他错了。
从标点上说, but 之后一般不得使用逗号, 但 however 位于分句之句首时, 通常用逗号, 位于分句之中时, 通常在其前后各加一个逗号, 位于分句之尾时, 则必须在其前加逗号。