although和but不能共用的原因是什么

Although用法：

1.although较正式，语气强
Although he was tired, he went on working.
2.although引导的从句放在主句前后均可，有时还可放在句中。
Although many difficulties are still ahead, we are determined to make greater achievements. 尽管在前面的道路上还有许多困难，但是，我们决心要取得更大的成就。
He often helps me with my English although he is quite busy. 尽管他相当忙，但是还常常帮我学英语。
3.although引导的从句不能与but, however连用，但可与yet, still连用。不能说：Although he was old, but he worked hard. 应把but去掉。当然，保留but而去Although也可。
注意:although 的几不:
although不能指假设的情况
although不能作并列连词
although不能作副词，放在词尾

英语语法中两个极其重要的原则：

1.英语的句子有且只有一个主干，主干的核心结构是主谓宾。

2.介词，连词，关系副词，关系代词等属于一种标记，它们标记着一些背景信息，属于搭架在1中主干上的枝叶。