left empty by consumers shifting their purchases to the web 这部分整体是一个过去分词短语作定语，修饰前面的二个名词shopping malls和retail stores。过去分词表示的是被动意义。在主动句中 Consumers left shopping malls and retail stores empty. leave在此是使役动词，消费者使购物中心和零售商店空若无人。形容词empty为宾语补足语。句子改为被动语态则为 Shopping malls and retail stores were left empty by consumers. 被动句中动词的逻辑宾语做了主语，动词改用被动语态，宾语补足语改称主语补足语，动作的施事者由介词by引出，作方式状语中的施事状语。需要注意的是虽然名称上叫做宾语补足语或主语补足语，但其实补足语是补足动词的语义不足，而不是不足名词的语义不足。所以宾补和主补都是动词的附加语。所以，在过去分词短语中，left empty by consumers 这个作定语的过去分词短语中，中心词是过去分词left，语法上和名词产生修饰关系的只是left，形容词empty和介词短语by consumers都是left的补足成分和修饰语，语法上和被修饰名词没有关系，尽管语义上有联系。所以，在分析名词的定语时，把left empty by consumers作为一个整体作定语，不单独分析empty。只是在分析这个过去分词短语的构成时，才把empty分析为left的主语补足语。

网友的整个句子结构分析如下：

A possible answer in crowded California cities 主语 is 系词 （to build housing on the growing number of shopping malls and big box retail stores left empty by consumers shifting their purchases to the web, which already have ample parking and are close to existing neighbourhoods）括号里整体是一个不定式短语作表语.

不定式短语内部结构分析如下：

to build及物动词 housing宾语 （on to build housing on the growing number of shopping malls and big box retail stores left empty by consumers shifting their purchases to the web, which already have ample parking and are close to existing neighbourhoods ）括号内为介词短语作地点状语。其中介词on的宾语为the growing number of shopping malls and big box retail stores。这二个并列的复数名词被过去分词短语left empty by consumers shifting their purchases to the web 和非限制性定语从句which already have ample parking and are close to existing neighbourhoods 所修饰。which的先行词为shopping malls and big box retail stores。