"Pela" is the contraction of "por" + "a". "Por" is like "by" and "a" is the definite article "the" in the female form. Example: "Joan passed by the girl without seeing her". In portuguese:"Joan passou pela garota sem vê-la". Did you understand?

"Para" in portuguese is like "to" in some phrases in English. For example: "They go to school next year" . The tranlation is: "Eles vão para a escola no próximo ano". So, "para" is a proposition of direction and motion. In some English phrases the preposition "to" isn't used, like in "He is going home". But, in portuguese, "para" is allways used: "Ele está indo para casa".

para has a transitive sense... para você (to you) - it's moving from someone else till you. para fazer isso (in order to do that) - changing from the previous situtation to the next one. por has an instrumental or motivational sense... por você (because of you) - you are the reason of why I'm doing something. por isto (because of this) - this is the cause of the mentioned effect. por este meio (by this mean) - through this mean, I do something. pelo/pela were "porlo / porla" in the past. They are in fact the union between "por" and an article "o / a". Look at this: Por o caminho.... (some centuries past till it became) Porlo caminho... (more centuries past and now we have) Pelo caminho. It follows our general rule of gender and number: pelo / pela pelos / pelas Pelo menos agora você tem um meio para estudar por mim! ;D

" para" 1. Expresses end term . 2 . Expresses place where ( with intention to delay ) . 3 . Expresses cause, motive . 4 . Expresses respect , comparison , fitness . 5 . Used to express intention or purpose .

I think with this example, you should can see better: This is the most usual: "Estou indo PARA a praia" , "I´m going TO the beach" "PARA você?","FOR you?" important: PÁRA!!!" com acento, It´s like "STOP THIS!!!" verb (parar - to stop)