POR (motivation or substitution): for - per , by -

. POR (motivation or substitution):

fuente : http://www.bowdoin.edu/~eyepes/newgr/ats/43.htm#

1. Exchange, substitution, rate, correspondence (per):

Te doy cinco dólares por el libro. Gracias por todo. No me tomes por idiota. a veinte kilómetros por hora el diez por ciento $3 por galón.

I'll give you five dollars for the book. Thanks for everything. Don't take me for an idiot. twenty kilometers an hour ten percent $3 per gallon

2.  a. Cause, reason, or motive of an action (because of, to fetch):

Lo merece por su dedicación. Fuimos al mercado por pan. Te quería por tu honestidad. No jugaron por la lluvia.

He deserves it for (because of) his dedication. We went to the store for bread. I loved you for your honesty. They didn’t play because of the rain.

b. On behalf of, for the sake of, in favor of, out of:

Lo hice por el dinero. No lo hago por ti. Se preocupan por mí. Luchan por la independencia. Trabajaba por la paz mundial. A menudo me pregunta por ti.

I did it for the money. I'm not doing it on your account. They worry about me. They fight for independence. She worked for world peace. He often asks about you.

3. Duration in time (often omitted):
Estuvo en México (por) seis semanas.He was (stayed) in Mexico for six weeks.

During the morning, evening, or afternoon:Trabajaba por la mañana.He used to work  in the morning.

4. Movement through a place:

Iba por la calle cuando... Pasa por mi oficina. Vamos a pasear por el parque.

I was going down the street... Drop by my office. We are going to take a walk through/by the park.

5. Agent in a passive phrase (by):

Fue escrito por ella. Está afectada por la noticia.

It was written by her. She's affected by the news.

6. Means of transportation or communication (by):

El paquete llegó por avión. Llámame por teléfono.

The package arrived by plane. Call me on the telephone.

7. In a large number of idiomatic expressions:

por fin por ejemplo por desgracia por supuesto por eso por aquí

finally, at last for example unfortunately of course that's why around here

por lo menos por favor por primera vez por casualidad por otra parte por lo tanto

at least please for the first time by (any) chance on the other hand therefore

 The questions ¿para qué? (for what purpose?) and ¿por qué? (for what reason or motive?) can give you clues on choosing the appropriate preposition. Both prepositions will often be grammatically correct, but they would convey different meanings:

Recibimos dinero por la investigación. I got money from the research (done). [exchange]	Recibimos dinero para la investigación. I got money for the research (to do). [purpose]
Compré esto por ti. I bought this because you wanted me to. [motive]	Compré esto para ti. I bought this to give it to you. [destination]