Both por and para translate to "for" in English, which is exactly why they're so easy to mix up. They're not interchangeable, though -- por looks backward at the cause, the means, or the exchange behind something, while para looks forward at the purpose, the destination, or the deadline it's aimed at.
Por points backward -- the cause, the means, the exchange behind something. Para points forward -- the purpose, destination, or deadline something is aimed at.
Cause, means, exchange, motion through
Purpose, destination, deadline, recipient
Trabajo por mi padre cuando está enfermo, pero normalmente trabajo para mi tío.
I work in my father's place when he's sick, but normally I work for my uncle.
Por mi padre means filling in on his behalf; para mi tío means he's the employer. Same sentence shape, opposite relationship to the person named.
Take the quiz and find out if por and para have started to feel different.