Spanish Preposition:por and para

"Por" and "para" have a variety of meanings, and they are often confused because they can each be translated as "for."

Even though your trusty Spanish-English dictionary will list for as the definition of both por and para, their use goes well beyond the direct English translation. It’s all about context. Even in English, while we may use for in multiple instances, it can convey a different meaning.

Examples:
Estoy buscando aquel libro para ti. (I am looking for that book for you.)
Estoy buscando aquel libro por ti. (I am looking for that book for you.)

In the first sentence, I am looking for that book to give it to you. In the second sentence, I am looking for that book on your behalf (because you may be too busy to go to the bookstore).

It’s a subtle difference, but understanding this in English will help you understand when to use por and when to use para in Spanish.

To learn to use "por" and "para" correctly, you need to do two things:

Learn the rules for how por and para are used.
Memorize model sentences.

Basically:
por → cause
para → effect

"Por" has many uses, and so it is the more problematic of the two.

• Motion/place: Caminan por las calles. (They walk through the streets.)
• Means/manner: Lo envío por correo aéreo. (I’m sending it by air-mail.)
• In exchange for/substitution: Voy a hacerlo por tí. (I’m going to do it for you.)
• Duration of an action: Trabajo por una hora. (I’m working for an hour.)
• Indefinite time period: Duerme por la tarde. (He sleeps in the afternoon.)
• On behalf of: La firmo por Ud. (I am signing it on your behalf.)
• Per: Me pagan por día. (They pay me per day.)
• To express gratitude or apology: Gracias por la ayuda. (Thanks for the help.)
• For multiplication and division: Dos por dos son cuatro. (Two times two equals four.)
• For velocity, frequency and proportion. Voy al restaurante cinco veces por semana. (I go to the restaurant five times per week.)
• Meaning "through," "along," "by" or "in the area of" Andamos por el parque.
• (We walk through the park.)
• When talking about exchange, including sales. Él me dio diez dólares por el libro.(He gave me ten dollars for the book.)
• To mean "on behalf of," or "in favor of,". No voté por nadie. (I didn't vote for anyone.)
• To express a length of time.Yo estudié por dos horas. (I studied for two hours.)
• To express an undetermined, or general time, meaning "during". Se puede ver las estrellas por la noche. (One can see the stars during the night.)
• For means of communication or transportation. Prefiero viajar por tren y hablar por teléfono. (I prefer to travel by train and speak by phone.)
• In cases of mistaken identity, or meaning "to be seen as". Me tienen por loco. (They take me for crazy.)
• To show the reason for an errand (with ir, venir, pasar, mandar, volver, and preguntar). Paso por ti a las ocho. (I'll come by for you at eight o'clock.)
• When followed by an infinitive, to express an action that remains to be completed, use por + infinitive. La cena está por cocinar.(Dinner has yet to be cooked.)
• To express cause or reason. El hombre murió por falta de agua. The man died for lack of water.
• "estar por" means to be in the mood, or inclined to do something. Estoy por tomar café. (I'm in the mood for drinking coffee.)
• In passive constructions. El libro fue escrito por Octavio Paz.(The book was written by Octavio Paz.)

"Para"

• Destination/person: Esto es para Ud. (This is for you.)
• Destination/place: Salimos para Madrid. (We are leaving for Madrid.)
• A future time limit: Es para mañana. (It’s for tomorrow.)
• Purpose/goal: Nado para divertirme. (I swim to have fun.)
• Use/function: Es un cepillo para el pelo. (It’s a hair brush.)
• Comparisons: Para su edad, lee bien. (For her age, she reads well.)
• Opinion: Para mí es demasiado crudo. (For me it’s too rare.)

Exercice Para/Por
http://www.learnspanishfeelgood.com/prepositions-para-por1.html