When I thought about how to answer this question I realized it probably seems really hard as there is not a real rule as to when to use which. I guess we just got used to it and know it by heart. I did however found something that might be of help to you. I don't know if we're allowed to paste links to other sites but here: http://www.spanishdict.com/answers/100035/para-and-por If it's not allowed please excuse me and this comment can be deleted. Anyway, hope that helps, it's more of a "Does this sound right?" kind of thing. Also the meaning can vary just by swapping them. "Lo hice por ella" and "lo nice para ella" both can be translated as "I did it for her" but one means "I did it for her cause I love her" (or alike) and the other means "I did it for her, because she doesn't know how to". It's really easier to just go check out that site than for me to try and explain this, it'd just end up being a full on 3 pages of examples and neither of us wants that hehehe. Good luck and hope it helps.

Estas dos preposiciones crean dificultad para algunos extranjeros que en su idioma no tienen esta diferenciación (como en inglés "for"). "PARA" se utiliza para expresar "finalidad" (este diccionario es PARA aprender español), "destino"/"destinatario" (este diccionario es PARA mi), "opinión" ("Para mí, este diccionario es muy malo")... "POR" se usa para expresar "causa" ("no leo más este diccionario POR aburrimiento"), "medio"/"modo" ("he recibido este diccionario POR email"), "sustitución" ("he cambiado otro libro POR este diccionario")... Estos son sólo algunos ejemplos para que tengas una idea de sus utilizaciones. Creo que la práctica es la que te hará que consigas alcanzar el uso correcto de ambas preposiones.

There is a general rule, I guess you are talking about the word "for" in English. Por = "for" for past/present Para = "for" for future(always future) For example, in the case of "I did it for her" when talking about a cake you just baked. You would use "para" when you refer to something that would act or serve in the future, like she eating the cake. ( future from the moment of the action ) You would use "for" when you talk about something present, like your current reason or the existing love for her or so. I hope that messy doubt is clearer now, you could post an example where you have doubts on it :P

Your question is too general, but I pasted a link here to help you. http://www.studyspanish.com/lessons/porpara.htm