nekare

hehe, parking would be 'Prohibido estacionar' (people in Monterrey say 'parkear', which is the trashiest thing ever. *eyeroll* they're so close to the border that they're losing their identities) and swimming would be 'prohibido nadar'. ('se prohibe' is correct, but in signs and stuff on the street (I'm guessing that's what it's for?) 'prohibe' is usually more used.)

Mmm, how do I explain it...

'para' indicates both a function ('es para beber' it's for drinking) and an origin ('es para tí' it's for you).

'Por' can also indicate an origin, athough it's more specific ('este libro fue escrito por jk rowling' this book was written by jk rowling). It's main function, though, it's to explain something, it's something like an abreviated version of 'porque' (because) like in: 'me fue mal por no haber estudiado' (I didn't do well because I hadn't studied).

note: it's not used in every single case, and it depends of the sentence like:
'me caí por el agua en el suelo' (I fell because of the water on the floor) is very different than
'me caí porque había agua en suelo' (I fell because there was water on the floor)
Er, that, you'll learn later, I'm sure. It's the sort of thing that comes easily once your nearly fluent in a language, when you know certain things go in a way because it sounds right. *shrug*

Um, hopefully that made sense. o_O Hehe, have you sung the prepostion song already? XD And don't you guys have something like it in English? (there's a pretty funny song for te form in Japanese...)