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[1] Vanity is a constant; [2] people will only start shopping more sustainably [3] when they can’t afford not to。
本句分号前后[1][2]是并列关系；[3]是状语从句
[1] Vanity is a constant这里理解为：追求浮华是人之常情
[2] people will (only强调后文的when从句，在此处不作翻译) start shopping more sustainably；这里理解为：人们才会开始可持续地消费
[3] (only) when they can’t afford not to (start shopping sustainably)；分句[2]中的only在强调从句[3]，宾语not to是一个不定式短语，该短语是省略形式，其完整形式为not to (start shopping more sustainably)。可以理解为:人们只在当他们不能承担放纵购物的时候，才会开始节制购物。