I. Cause and effect

Por expresses the motive for an action or the agent performing the action. Para expresses the goal of an action or the recipient of the action. Por points back towards the cause (); para points toward the effect ().


Por (←)Para(→)
Lo mataron por odio.
They killed him out of (motivated by) hate.
Ella estudia para profesora.
She is studying (in order) to become a professor.
Todo lo que hago, hago por ti.
Everything that I do, I do for you (on your behalf).
Él lo hizo para sobrevivir.
He did it (in order) to survive.
El libro fue escrito por Stephen King.
The book was written by Stephen King.
Estos son juegos para niños.
These are games (intended) for children.
Ellos mandaron por el médico.
They sent for the doctor (motive of the call).
El libro es para usted.
The book is for you.
Mamá fue a la tienda por leche y pan.
Mama went to the store for milk and bread (motive of an errand).
Es una taza para café.
It`s a coffee cup (a cup intended to be used for coffee).

*Also note that parsa is sometimes placed in front of an infinitive to mean in order to. This meaning , often understood from context in English, must always be expressed in Spanish:
Estamos aquí para estudiar.   We are here (in order) to study.

II. Movement through versus movement toward:

To express movement in space and time, para retains its basic meaning of movement toward an objective(-->\). Por takes on a different meaning of duration or movement through space or time with no destination specified  (-\->).
Por Para
Javier va por el pueblo.
Javier passes through the town.
Javier va para el pueblo.
Javier heads toward the town.
Estaremos en clase por la mañana.
We will be in class during the morning.
Termínenlo ustedes para mañana.
Finish it by tomorrow.
Pepa estará en México por tres días.
Pepa will be in Mexico for (a period of) three days.
Pepa estará en México para el tres de agosto.
Pepa will be in Mexico by the third of August.

*Note that many speakers of Spanish use no preposition at all to express duration of time:
Pepa estará en México tres días.  Pepa will be in Mexico (for) three days.

III. Other uses

Por and para also have uses that do not fit into the preceeding categories. Por expresses in exchange for or per in units of measurement, as well as the means by which an action is performed.

Por Para
Te doy cinco dálares por el libro.
I`ll give you five dollars (in exchange) for the book.
Para expresses in comparison with and also in the opinion of.
Para (ser) perro, es muy listo.
He is smart for a dog.

Para ella, la fe tiene mucha importancia.
For her, (her) faith is very important.
Lo mandaron por avión.
They sent it by plane.

El camión sólo corre 20 kilómetros por hora.
The truck only goes 20 km per hour.
*See me for any more questions.


Remember that the prepositions that follow some English verbs are expressed by the simple verb form in Spanish:

buscar = to look for pagar= to pay for
esperar= to wait for pedir= to ask for

*However, to ask after or about someone is expressed with a preposition: preguntar por.   Ellos preguntaron por ti en la fiesta.   They asked about you at the party.

Test yourself--- take the Por vs Para Online Quiz at http://www.quia.com/jq/17705.html