你可以从句子本身去理解 People will only start shopping more sustainably when they can't afford not to. 补全not to之后 People will only start [shopping more sustainably] when they can't afford not to [shop more sustainably]. 你把[]中的当成X 当人们不去X + 他们钱不够 when can't afford not to X 他们才会X only start X