Por and Para
In general, por relates to the origin of things, whereas para refers to their final destination. Por expresses where things originate and para expresses where they are headed:

EXAMPLE
El documento fue preparado por el consejo para el rey.	The document was prepared by the council (its origin) for the king (its destination).

There are parallels when each of these prepositions is used to designate time and place. Para once again refers to aim or destination:

EXAMPLE
Debemos terminarlo para mañana.	We must finish it by (or for) tomorrow.
Mañana salgo para Sevilla.	Tomorrow I leave for Seville.

In contrast por designates time ‘during which’ or place ‘through which’ something occurs:

EXAMPLE
Vamos a París por un mes.	We are going to Paris for a month.
Va a Sevilla (pasando) por Madrid.	He is going to Seville via Madrid.

Unfortunately, it is not possible to establish clearly contrasting uses for most of the contexts in which por and para are used.


Uses of por
1. Por indicates the agent ‘by whom’ or ‘by which’ an action is initiated, thereby introducing the agent or ‘doer’ in passive constructions:

EXAMPLE
Fue invadido por los romanos.	It was invaded by the Romans.

2. Por indicates the reason or cause which initiated an action. In English this is frequently ‘because of’, ‘on account of’, ‘through’, ‘for’:

EXAMPLE
Me gusta por lo fácil que es.	I like it because of how easy it is.
Lo admiro por su paciencia.	I admire him for his patience.
Por estar enferma no pudo venir.	Through being ill she could not come.

3. Por denotes time during or throughout which an action takes place, often English ‘for’:

EXAMPLE
Piensa venir por un mes.	He intends coming for a month.

4. Por indicates a place through which movement takes place, ‘through’, ‘via’ or ‘along’: 

EXAMPLE
Tuvimos que salir por la ventana.	We had to go out via the window.
Caminaba por la selva.	I was walking through the jungle.

5. Por denotes the manner or means by which something is done, ‘by’ or ‘by means of’:

EXAMPLE
por teléfono/carretera/correo electrónico	by phone/road/email

6. Por refers to the exchange of one thing for another:

EXAMPLE
Lo compré por 500 euros.	I bought it for 500 euros.
Cambió su regalo por otro.	She changed her present for another.

7. Por states ‘on behalf of’, ‘in favour of’, ‘in support of’:

EXAMPLE
No lo hagas por mí.	Don’t do it just for me.
El está aquí por Andrea.	He’s here on behalf of Andrea.
Estoy por los derechos indígenas.	I am in favour of indigenous rights.

8. Por expresses a description or designation that is a personal opinion or evaluation (sometimes English ‘for’ or ‘as’):

EXAMPLE
Lo tomé por otro.	I took him for someone else.
Podrían pasar por hermanas.	They could pass for sisters.

9. Por translates rate and quantity:

EXAMPLE
Viene siete veces por semana.	He comes seven times a week.
Es más económico comprarlos por kilo.	It is more economical to buy them by the kilo.

10. Por followed by an infinitive indicates what remains to be done:

EXAMPLE
Me quedan tres páginas por fotocopiar.	I have three pages left to photocopy.
Está todo por discutir.	Everything is still open for discussion.

Uses of para
1. Para indicates destination, with reference to a place, to a person as an intended receiver or beneficiary, or to a function:

EXAMPLE
Mañana salgo para Guayaquil.	Tomorrow I am leaving for Guayaquil.
unos libros para ti	some books for you
Están estudiando para el examen	They are studying for the examination.
¿Para qué sirve este instrumento?	What is this instrument for?

2. Para refers to a deadline or target in time, corresponding to ‘for’, ‘by’, ‘until’:

EXAMPLE
Para el domingo estará completo.	By Sunday it will be complete.
Tengo una cita para el sábado.	I have an appointment for Saturday.
Aplazó la charla para la semana que viene.	He put off the talk until next week.

3. Para followed by an infinitive expresses purpose, corresponding to English ‘(in order) to’:

EXAMPLE
Se necesita mucho dinero para hacerlo.	You need a lot of money to do it.
Lo dijo para impresionar a la gente.	He said it in order to impress people.
un trapo para limpiar cristales	a cloth for cleaning windows

Por and para with estar – comparisons
In Spain this generally indicates what is about to be done, or is on the point of being done, what might otherwise be expressed as estar a punto de:

EXAMPLE
Estoy para salir.	I am about to leave.
El tren está para partir.	The train is on the point of departing.