The por and para question always seems to put everyone in a bad mood. And that's because it's one of those conceptual grammar rules that depends on situation, and doesn't make much sense. Why are there two words (actually, prepositions) to say essentially the same thing? And, what exactly does POR in Portuguese really mean?
Eu não sei. Nem quero saber!
The road to happiness is as always : simplify. Let's learn the TWO MOST COMMON uses for each.
Note: Por very, very often gets used along with "the" (o, a) and when it does it gets CONTRACTED like this:
por + o = pelo
por + a = pela
I'm sure there's an explanation for this. But it's not an obvious contraction and, I think it's that way because it sounds good: PAY-LOW & PAY-LA.
Now combine por + o,a:
Now using the contractions (above):
Now with the contraction:
Para is used always to say in order to do something or go somewhere.
And if you're really LIGADO com a língua and wonder if you can also contract para + o,a.
But it's (for now) just coloquial, which means that you should only use it in informal, spoken situations. Language evolves, and one day these contractions may be adopted into the official lexicon:
para + o = pro
para + a = pra