快速答案
faltar 表示距需求还缺多少；sobrar 表示使用后多余多少。
可靠的方法是先判断这个词在句子里的功能，再翻译。
每个词的作用
faltar
to be missing or insufficient
Use faltar for a needed amount, item, person, or time not being present.
sobrar
to be left over
Use sobrar for an amount remaining beyond what was used or needed.
如何选择
- faltar: Choose faltar if the sentence points to a needed amount, item, person, or time not being present.
- sobrar: Choose sobrar if the sentence points to an amount remaining beyond what was used or needed.
例句
Faltam dez reais para pagar a conta.
We are ten reais short of paying the bill.
Sobrou comida para amanhã.
There was food left over for tomorrow.
常见误区
Portuguese often puts the quantity as the subject: faltam dez minutos and sobraram duas vagas.
放到语境中练习
从你的真实生活中选一个句子，写两遍：一次用 faltar，一次用 sobrar。如果两个版本都可以，说明意思怎样变化。