快速答案
por 侧重原因、方式或经过的路径；para 侧重目的地、接受者或目的。
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
por
through, by, because of, or for
Use por for cause, means, exchange, route, duration, and action on someone’s behalf.
para
to, toward, or in order to
Use para for destination, recipient, purpose, goal, and a future deadline.
如何选择
- por: Choose por if the sentence points to cause, means, exchange, route, duration, and action on someone’s behalf.
- para: Choose para if the sentence points to destination, recipient, purpose, goal, and a future deadline.
例句
Enviei o arquivo por e-mail.
I sent the file by email.
Este presente é para você.
This gift is for you.
常见误区
Both often translate as “for.” Ask whether the phrase explains why or how, or points to where, whom, or what goal.
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 por，一次用 para。如果两个版本都可以，说明意思怎样变化。