同学你好，该知识点来自沪江网校《2022考研专业课名师联报班【306西医综合+政英】（渠道专享）》的课程，想要更系统的学习，欢迎进入课程学习。不仅可以和更多的同学一起学习，而且还有老师、助教随时的学习指导和知识点解答哦。

该句中含有插入语：If it's message confined merely to inform action -- and that in itself would be difficult if not impossible to achieve,for even a detail such as the choice of the color of shirt is subtly persuasive--advertising wound be so boring that no one would pay any attention.
我们先把插入语省去，分析句子，然后还原插入语，这样能更清晰地分析句子结构。
去掉插入语得到：If its message were confined merely to information, advertising would be so boring that no one would pay any attention. 如果广告的内容仅限于提供信息，那么广告会令人生厌以致无法获得关注。
我们再看插入语部分 。
and that in itself would be difficult/ if (it is) not impossible to achieve, for even a detail such as the choice of the color of shirt is subtly persuasive. 此处that为代词，指代its message were confined merely to information. for 表示原因。该部分翻译为：虽然这有可能达成，但它本身也是很难做到的，因为诸如挑选衬衫夜色这样的细节也是具有一些劝说性的。