快速答案
por 无重音，是介词；pôr 加抑扬符，是“放置”这个动词。
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
by, through, or for
Use por for the unaccented preposition expressing route, cause, exchange, agent, and other relations.
pôr
to put
Use pôr for the infinitive verb meaning place or set.
如何选择
- por: Choose por if the sentence points to the unaccented preposition expressing route, cause, exchange, agent, and other relations.
- pôr: Choose pôr if the sentence points to the infinitive verb meaning place or set.
例句
Passamos por Belo Horizonte.
We passed through Belo Horizonte.
Vou pôr os livros na estante.
I am going to put the books on the shelf.
常见误区
Conjugated forms such as põe and puseram reveal the verb. The infinitive accent distinguishes it from the preposition.
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 pôr。如果两个版本都可以，说明意思怎样变化。