Para y Por
There have been a few examples using both para and por, but no explanation of any kind regarding how you know which one to use. So how do you know?
"para" is to express causality. Like "I'm doing homework FOR school" - "Estoy hacendo mis deberes para la escuela". "por" is more general. Like: "Pay for her" - "Paga por ella" or to help ask questions "Why don't you pay" - "Por qué no pagas?"
Read this, they explain pretty well http://spanish.about.com/cs/grammar/a/porpara.htm
The best way that I heard it explained is that if someone or something is receiving or benefiting, it's generally para, otherwise por. Their example was "I'm shopping for my mother." If you are buying groceries to give to her, it's para. If you are shopping on her behalf, it's por. I'm studying for school: para. Two for one: por. It's not perfect but it it's a good way to guess! :)
There was a phrase "ella consulta para mi padre", which according to this website could only be translated at "she consults for my father". Clearly para means for, but it does not really make sense in English! It is rather silly that when I translated it as "she consults my father", it showed it as incorrect