What is the difference between por and para?
Para points forward to a destination, purpose or deadline — the thing you are heading towards. Por points backwards or around: the cause, the route, the exchange, the duration. Both come out as for in English, which is precisely why translating the English word is the thing that keeps getting people wrong.
Para looks forward
If you can rephrase the sentence with in order to, for the benefit of, or by the time that, you want para. It marks the endpoint: where something is going, who it is for, what it is meant to achieve, or when it is due.
Salgo para Madrid.
I'm leaving for Madrid.
Destination.
Es para ti.
It's for you.
Recipient.
Estudio para aprobar.
I study in order to pass.
Purpose.
Para el lunes.
By Monday.
Deadline.
Por looks backward or around
Por covers cause, means, exchange, duration and movement through. If you can rephrase with because of, by way of, in exchange for, or during, you want por.
Gracias por la ayuda.
Thanks for the help.
Cause — what prompted the thanks.
Pasé por el parque.
I went through the park.
Route.
Lo compré por diez euros.
I bought it for ten euros.
Exchange.
Estudié por tres horas.
I studied for three hours.
Duration.
The pair that flips the meaning
Some sentences are grammatical with either, and the meaning changes entirely. Trabajo para Juan means Juan is my employer. Trabajo por Juan means I am covering his shift. Neither is a mistake; they are different statements, and the preposition is doing all the work.
Trabajo para Juan.
I work for Juan (he's my boss).
Trabajo por Juan.
I work in place of Juan (covering for him).
Common mistakes
Gracias para tu ayuda.
Gracias por tu ayuda.
Thanks is a reaction to a cause, so por. This is among the most common single errors learners make.
Estudio por aprobar.
Estudio para aprobar.
Passing is the goal you are aiming at, so para.
Salgo por Madrid.
Salgo para Madrid.
Madrid is the destination. Por Madrid would mean travelling around inside the city.
Frequently asked questions
Is there a memory trick for por and para?
The most useful one is direction: para is an arrow pointing at a target, por is everything surrounding the action. Purpose, destination and deadline all point forward and take para; cause, route, exchange and duration all take por.
Why do both mean for?
Because English uses one preposition where Spanish uses two, and the split does not run along any line English marks. This is why translating from the English word fails — you have to decide what relationship you actually mean before choosing.
Which one is used more often?
Por, by a clear margin, largely because it covers cause and duration which come up constantly. But para is more predictable, so learning its four uses first and treating por as the default for everything else is a workable strategy early on.