por and para are two prepositions, in eng. we only use one "for". thats why it becomes harder to understand the different uses. por is used when expresing time, duration, cause, movement, transportation and some other expressions.
time: por la tarde, in the afternoon
duration: por dos horas, for two hours
cause: lo hize por ti, i did it because of you
movement: camine por la calle 5, i walked by 5 st.
para is used when it means, in order to, directed to, direction, specific time.
in order to: para estudiar necesitas el libro, in order to study you need the book
directed to: la carta es para usted, the letter is for you
direction: me voy para Europa, I am goint to Europe
specific time: lo necesito para la tarde, I need it for this afternoon
these two prepositions have some other uses,but these are the main ones. also por is used in phrases like por favor , please, por ejemplo, for example.