When one thing is exchanged for another, por is used to express “for,” as in when money is exchanged for something.
Talia pagó veinte dólares por los zapatos.
Talia paid twenty dollars for the shoes.
Quiero darte mi coche por tu coche.
I want to give you my car (in exchange) for your car.
Whether or not you realize it, when you thank someone, you are exchanging your words of gratitude for something that has been done for you or given to you. For this reason, por is used after gracias or the verb agradecer (to thank).
En España, los niños les agradecen a los Reyes Magos por los regalos.
In Spain, the children thank the Three Wise Men for the gifts
Te doy las gracias por tu ayuda.
I give you thanks for your help.
It is difficult to differentiate between motivation (which is expressed with por) and purpose (which is expressed with para). Motivation is the equivalent to the English expression “due to.” If you could replace the Spanish preposition with “due to,” then the Spanish preposition should be por. Motivation is the cause behind it rather than the goal in front of it. The following examples show the different uses of por and para:
Tengo muchas muestras de champú por el viaje de negocios.
I have many samples of shampoo due to the business trip.
Tengo muchas muestras de champú para el viaje.
I have many samples of shampoo for the trip.
There are also a few verbs that are consistently followed by the preposition por to indicate motivations. Andar (to walk), caminar (to walk), ir (to go) , regresar (to return), volver (to return), enviar (to send), and similar verbs will be followed by por when the purpose or motive is being provided.
Ellos van a la tienda por la leche.
They go to the store for milk.
Raul regresó por la comida de su madre.
Raul returned for his mother's food.
Means (of communication or transportation)
When you want to indicate a means of communication or transportation, por is used. Here are some typical expressions with por indicating means of communication and transportation:
Movement (in an area)
While it's misleading to translate an English preposition to a Spanish preposition, you should use por anytime the idea expresses “through,” “by,” or “along.” Por indicates movement within an area.
Viajamos por los pueblos blancos de la Costa del Sol
We traveled through the whitewashed villages of the Costa del Sol.
Caminamos por las orillas del río.
We walked by the bank of the river.
Ella sigue por la calle principal.
She continues along the main street.
The best way to know that the preposition is indicating substitution is to try the English expression “in place of” or “instead of.” If these expressions sound appropriate, the preposition is indicating a substitution, which requires the use of por. Compare the following pairs of sentences:
Isabel compra el regalo por su madre. Su madre está enferma y no puede ir de compras. Isabel buys the gift for (in place of) her mother. Her mother's ill and can't go to the store.
Isabel compra el regalo para su madre. Es el cumpleaños de su madre.
Isabel buys the gift for her mother. It's her mother's birthday.
Marta canta para Yolanda. A Yolanda le gusta la música.
Marta sings for Yolanda. Yolanda likes music.
Marta canta por Yolanda. Yolanda no puede cantar porque tiene laringitis.
Marta sings in place of Yolanda. Yolanda can't sing because she has laryngitis.
There are certain verbs that use the preposition por to create an idiomatic expression and certain verbs that are used with para. These do not logically follow any of the rules for using por and para, because that is what an idiomatic expression is: a group of words that have a specific meaning when used together but don't make sense if you try to translate them separately into English. See Appendix
for a list of idiomatic expressions. There are many that include either por or para.
To avoid confusing the por and para rules, learn the idiomatic expressions with por and para separately. It may help you learn them if you put all the idiomatic expressions that include the preposition por on one color of flashcard and the expressions using para on another color, so that the color can help you remember the appropriate preposition. When you quiz yourself, rewrite both sets of idiomatic expressions on all white cards to see if you can remember whether to use por or para with each expression, as well as to see if you understand what the expression means.