Para vs. Por

Can someone explain the difference between the use of "para" and "por"? It seems at times they are interchangeable but I know they aren't. I just haven't been able to figure out the rule, if there is one. I'm working my way through the lessons; I'm in the beginner level, 4.5. Thanks!

http://www.studyspanish.com/lessons/porpara.htm

In addition to the web page LarryD has recommended, here are three more that you may find useful: * __http://www.bowdoin.edu/~eyepes/newgr/ats/43.htm__* - This excellent page from Bowdoin College includes some exercises and, at the end, under #41 under Más Práctica, links to a number of other pages dealing with por and para. *__http://www.study-spanish-language.com/spanish-lesson-por-para.htm__ * and, of course, *__http://www.rocketlanguages.com/spanish/spanish_prepositions_3.php__*

Thanks!!

I'm on section 4 of RS2 advance course and I still strugle with por vs pora. One noticeable pattern that I’m noticing is that pora seems to be used often, when referring to people, places or things. Por seems to used most often when referring to ideas or thoughts. For example, based on what my gringo ears have been able to pickup, I would probally say somethig like. "pora mi, prefiero estudiar mi espanol en la noche" or "garacias por todos mi amigo" Of course I could be wrong but I think over time your ears will begin picking up certain patterns.