核心问题
Although DDT was first produced in 1874, but it was not widely used until the second quarter of this century. (TOEFL) 我看不出上面的句子有什么错误。
词典解答
你之所以看不出上面的句子有什么错误,是因为你受了汉语的干扰:上面的错句与汉语的表达方式恰好相同:虽然……但是。然而在英语里,在从属连词 although(though)之后绝不可使用并列连词 but。至于在 although(though)之后使用 yet, nevertheless 等副词,则属于另外的情况(参见 though...yet)。
although/though