Another difficulty for English speakers is the use of por and para in Spanish. A variety of English words are used to translate the two: for, at, over, on, out of, due to, as, in, around, through, by and to, beeing the most difficult the translation of for.

Para

Use para to translate the English for, by and to to express:

Destination:

In this specific use, para is almost equivalent with a:

We shall leave for Mexico City tomorrow.	Saldremos para La Ciudad de México mañana.
We moved to the new house in December.	Nos mudamos para la nueva casa en diciembre.

Recipient or person for whom something is intended:

Here is a book for you.	Aquí tengo una carta para tí.
I don't plan to do anything for the teacher.	No pienso hacer nada para el profesor.

Deadline:

I want you to do the homework by Friday.	Quiero que hagan la tarea para viernes.
We have to be done by tomorrow.	Tenemos que terminarlo para mañana.

Goal, intended goal or purpose:

 The English equivalent for this use is to or in order to.

I am working a lot to make money.	Trabajo mucho para ganar dinero.
I am studying to be a doctor.	Estudio para médico.	Equivalent to "Estudio para ser médico".

An unexpected comparison:

You speak Spanish very well for a foreigner.	Usted habla español muy bien para ser extranjero
For a teacher, he doesn't have much patience.	Para profesor tiene poca paciencia.
He knows a lot for his age.	Sabe mucho para su edad.	Notice that you can also say: "Sabe mucho por su edad.", but then the meaning of the sentence would be: "He knows a lot because of his age.", something completely different.

That the information in a sentence is someone's opinion:

In my opinion, the price is good.	Para mí, el precio es bueno.
In the opinion of the children, the parents are never right.	Para los hijos, los padres nunca tienen razón.

That something is about to happen:

The bus is about to leave.

El autobús está para salir.

English only to or not/ never to: 

The sun came out only to be hidden later in the clouds.	Salió el sol para esconderse luego en las nubes.
He went off to Mexico never to return.	Se fue a México para no volver.

Expressions with "para":

para entonces	by that time
para otra vez	for another (later) occasion
para siempre	forever
para variar	just for a change
ser tal para cual	to be two of a kind

Por

 Use por to translate the English for, by, to, as, in, around, through, at, over and per to express:

 Duration of time, imprecise point in time or distance. English for, around, in:

She worked in an assembly plant for many years.	Ella trabajó en una maquiladora por muchos años.
We'll see each other around Christmas.	Nos veremos por Navidad.	Imprecise point in time.
I saw her yesterday in the afternoon.	La vi ayer por la tarde.	Imprecise point in time.
We traveled for more than two hundred miles.	Viajamos por más de dos cientas millas.	Distance.

Exchange or Substitution ("on behalf of", "in place of"). English for:

I paid a lot of money for this computer	Pagué mucho dinero por esta computadora.
Juan did the job for me.	Juan hizo el trabajo por mí.	= "instead of" Juan.
He says that he does it all for his country.	Dice que lo hace todo por su país.

Occurences in a sequence. English for, by.

They left one by one.	Salieron uno por uno.
This is what he told me, word for word.	Eso es lo que me dijo, palabra por palabra.

The act of going for something or someone. The verbs most commonly used in this context are "ir", "venir", "mandar", "pasar" English for:

I came only for you.

Solo vine por ti.

The motion through something: 

In order to get to the dorm you must go through the tunnel.	Para llegar a la residencia tienes que pasar por el túnel.
The thief entered through the window.	El ladrón pasó por la ventana.
I got the job through my father.	Conseguí el trabajo por mi padre.

Cause or reason. English because of, out of, due to, at, over, on: 

We didn't go to the beach because of the rain.	No fuimos a la playa por la lluvia.
The girl fell because she didn't listen to her mother.	La niña se cayó por no escuchar a su mamá.
They fight over every little thing.	Se pelean por cada cosita.
Some students didn't have such a good time in Granada because of the cold in the houses.	Algunos estudiantes no lo pasaron tan bien en Granada por el frío en las casas.
I congratulate you on your birthday.	Te felicito por tu cumpleaños.

The means of ... English by, via: 

The students communicate with each other by email	Los estudiantes se comunican por correo electrónico.
We went to Mexico by plane.	Fuimos a México por avión.

The English term "times" in math problems: 

Three times three are nine.

Tres por tres son nueve.

The agent in a passive sentence. English by: 

The house was built by his father

La casa fue construida por su padre.

Expressions with por:

por ahora	for now
por cierto	certainly
por Dios	for heaven's sake
por eso	for that reason/ therefore
por favor	please
por fin	at least/ finally
por lo menos	at least
por lo visto	apparently
por supuesto	of course
por lo tanto/ por consiguiente	consequently
por lo que a mí me toca	as far as I am concerned
por lo pronto	for the time beeing
por lo mismo	for that very reason
por mi parte	as far as I am concerned
por una vez	for once
por todas partes	everywhere
por desgracia	unfortunately
por suerte	luckily, fortunately
por otra parte	on the other hand
por el estilo	something like that
por lo general	in general
por casualidad	by chance
por consiguiente	therefore
por ejemplo	for example
por si acaso	just in case
hay mucho por hacer	there is a lot left to do