¿Para o Por? ¿Cuál usar?

When to use por and when to use para? This is still somewhat of a confusing issue for me - sometimes I really have to think hard to choose the correct usage, other times it's a bit of a wild guess when I'm unsure of the usage bucket (concept) it falls into. However I do have it down at the nearby taco shop as I can usually say "para aquí" before they ask if I want for here or to go!

So here are the buckets that helped me sort the uses out - let's start by looking at the emytology of the root words that form para and por - por comes from the Latin root pro and means for - that's pretty straight forward. Think pro bono, meaning "for good". Para comes from pro plus ad, meaning to. So her goes:

The uses of Por:

1. Tiempo de límite - a time period (during, in, for) - Están aquí por una semana - They are here for a week. Trabajo por la noche - I work at night.
2. Por medio de - through or by means of (along, by, around) - Él pasó por su casa - He passed by your house. Llegué por autobús - I arrived by bus.
3. A cuasa de - because of (on account of, on behalf of) - No fui por el frío a Nueva York - I didn't go to New York because of the cold.
4. A cambio de - in exchange for - Pago 20 dólares para el libro - I paid $20 for the book.
5. En medidas - in measurements (by, per, for) - Sesenta millas por hora - Sixty miles per hour. Recibí un descuento de 20 por ciento - I got a 20 percent discount.
6. En busca de - in search of, for - Voy a venir por ti a las siente - I'll cll come by for you at seven.
   

The uses of Para:

1. Destino - destination - Salí para Arizona hoy - I left for Arizona today.
2. Futuro objetivo - future goal (by or for a certain time) - Quiero el coche para el lunes - I want the car for Monday. Debo estar allí para el mes de febrero - I must be there by the month of February.
3. Para quien o qué algo es - whom or what something is for - Compré una cama nueva para mi sala - I bought a new bed for my room. Él compra flores cada día para su madre - he buys flowers every day for his mother.
4. A fin de a - in order to - Él necesita más dinero para pagar el viaje a Mérida. - He needs more money inorder to pay fr the trip to Merida.
5. Proposito corriente - current goal - Mi esposa estudia para médica - My wife is studying to be a doctor.

Once you feel you've mastered the nuances, take this quiz at spanish.about.com