The people of the Netherlands, with a long tradition of fisheries and mercantile shipping, had difficulty in developing good harbors suitable for steamships:eventually they did so at Rotterdam and Amsterdam, with exceptional results for transit trade with Germany and central Europe and for the processing of overseas foodstuffs and raw materials (sugar, tobacco, chocolate, grain, andeventually oil). (TPO18, 59)托福听力评分标准

mercantile /'mɜːk(ə)ntaɪl/ adj. 贸易的，商业的

exceptional /ɪk'sɛpʃənl/ adj. 异常的，杰出的，突出的

我是分界线，大家先一遍速读看是否理解

The people of the Netherlands, (with a long tradition of fisheries and mercantile shipping),had difficulty (in developing good harbors suitable for steamships): eventually they did so at Rotterdam and Amsterdam, (with exceptional results) (for transit trade with Germanyand central Europe) and (for the processing of overseas foodstuffs and rawmaterials) (sugar,tobacco, chocolate, grain, and eventually oil). (TPO18, 59)

分析：

修饰一：(with a long tradition of fisheries and mercantile shipping) ，介词短语，修饰The people of the Netherlands

中文：有着长期渔业和商业海运传统

修饰二：(in developing good harbors suitable for steamships) ，介词短语

中文：在发展适合蒸汽船的好港口

修饰三：(with exceptional results) ，介词短语

中文：有杰出的成果

修饰四：(for transit trade with Germany and central Europe) ，介词短语

中文：因为和德国以及中欧的贸易

修饰五：(for the processing of overseas foodstuffs and raw materials) ，介词短语

中文：加工海洋产品以及原材料

参考翻译：

有着长期渔业和商业海运传统的荷兰人，在发展适合蒸汽船的好港口上有困难：最后他们在Rotterdam and Amsterdam这样做了，并因为和欧洲以及中欧的贸易以及加工海洋产品以及原材料(如糖、烟草、巧克力、谷物以及油)，从而取得了杰出的成果。

这个句子的主要修饰成分就是介词短语，大家务必想清楚，每一个修饰成分修饰的是什么，才能看清楚这个句子的结构。