Por vs para: a practical guide

Both por and para are often translated as "for" in English, which is why they cause so much confusion. A rough way to start: para points forward, to a goal, a destination or a recipient. Por points to a cause, a route, a means or a period of time.

Uses of para

Purpose, followed by an infinitive: Estudio para aprender. (I study in order to learn.)
Recipient: Este regalo es para ti. (This gift is for you.)
Destination: Salimos para Barcelona mañana. (We leave for Barcelona tomorrow.)
Deadline: Necesito el informe para el viernes. (I need the report by Friday.)
Employer or use: Trabajo para una empresa finlandesa. (I work for a Finnish company.)
Opinion or standard: Para mí, es difícil. (For me, it is difficult.) Para ser extranjero, habla muy bien. (For a foreigner, he speaks very well.)

Uses of por

Cause or reason: Llegué tarde por el tráfico. (I arrived late because of the traffic.) Gracias por tu ayuda. (Thanks for your help.)
Duration: Viví allí por tres años. (I lived there for three years.)
Exchange or price: Pagué diez euros por el libro. (I paid ten euros for the book.)
Movement through or along a place: Paseamos por el parque. (We walked through the park.)
Means of communication or transport: Lo envié por correo. (I sent it by post.) Viajamos por tren. (We travelled by train.)
Per (frequency): Voy al gimnasio tres veces por semana. (I go to the gym three times a week.)
Times of day, approximately: Por la mañana, leo el periódico. (In the morning, I read the newspaper.)
Substitution or on behalf of: Hablo por mi madre. (I am speaking on behalf of my mother.)

A side-by-side comparison

Por	Para
Trabajo por dinero. (I work for the money: the reason)	Trabajo para ganar dinero. (I work in order to earn money: the goal)
Compré flores por mi madre. (I bought flowers for my mum, on her behalf)	Compré flores para mi madre. (I bought flowers for my mum, as a gift)
Pasé por la farmacia. (I went past the pharmacy.)	Salí para la farmacia. (I set off for the pharmacy.)

The English word is the same in each row. The Spanish word depends on the idea behind it.

Useful fixed phrases

por favor, por supuesto, por ejemplo, por fin, por cierto (please, of course, for example, finally, by the way)
para siempre, para nada, para variar, estar para + noun (forever, not at all, as usual, to be in the mood for)

Common mistakes

Using para for cause. Gracias para tu ayuda is wrong. Say Gracias por tu ayuda.
Using por for a deadline. Lo necesito por el lunes is unnatural. Say Lo necesito para el lunes.
Using por for a destination. Salgo por Madrid sounds like I am leaving through Madrid. Use para for the destination.

Practice

Choose por or para for each sentence.

Once these feel automatic, the next challenge is mood. The subjunctive triggers guide covers another area where Spanish asks you to choose carefully.