More or less "por" would be "by" or "because" and "para" would be "for", but as you already know it is more complicated than that. Generally speaking, "por" implies a cause and "para" implies an intention.
"Para" is easier and more straightforward:
- Esto es PARA ti -> This is FOR you (I want them to be yours = intention)
- Lleva las manzanas PARA comer -> Take the apples TO eat [them] (FOR them to be eaten, I want you to take them so you can eat them = intention)
- Jugamos PARA ganar -> We play TO win (FOR us to win the game)
"Por" is more complicated. The basic usage means causation:
- Ha pasado POR tu culpa -> It was your fault (It happened BECAUSE of your fault = cause)
- Los abuelos vienen POR Navidad -> The grandparents come FOR Christmas (when it is Christmas)
"Por" is very often used in the phrases "por qué" (why) or "porque" (because)
- ¿POR QUÉ viniste? Vine PORQUE quise -> WHY did you come? I came BECAUSE I wanted to [come]
- Hace sol PORQUE es verano -> It is sunny BECAUSE it is summer
Also "por" is used to indicate multiplication:
- Tres POR cuatro, doce -> Three TIMES four [equals] twelve
Also "por" means "in case"
- Lleva el paraguas POR si llueve -> Take the umbrella IN CASE it rains
I had to think this over, but I hope it clears things somewhat. Try these examples:
- I drink to forget -> Bebo ____ olvidar
- I came to see a friend -> Vine ____ ver a un amigo
- It is for you because you won it -> Es ____ ti _____ tu lo ganaste
- I am dying for you -> Es ____ ti que yo me muero (it is because of you that I die)
- For whom the bells toll? -> ¿____ quien doblan las campanas? Hemingway!!!
- I'll change my 911 for your Mustang -> Te cambio el 911 ____ tu Mustang
