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这句话的主干是There was a very interesting remark.
in a bookey by an Englishman that I read recently是介词短语作后置定语.
介词短语作后置定语的标准格式是介词+名词.
giving what he though was a reason for this American characteristic是过去分词短语作Englishman的后置定语.
分词短语的标准形式是不是也是非谓语+宾语.
这里是一个what引导的宾语从句.