A summary of instruction on when to use POR or PARA in Spanish grammar
PARA
to express
an objective or goal which could be
an action (Carmen studies IN ORDER TO become a doctor)
a person (Carmen is working FOR Mr. Martinez)
a place (This is the bus FOR Madrid)
a point in time (You need to learn the lesson BY tomorrow)
POR
to express
duration (I'm am going to work in Spain FOR three months)
manner or means (Carlos talks to Clara BY telephone)
movement through or along a place (Enter THROUGH the main door.)
exchange (I'll give you my book FOR your watch.)
Go to this link for more information
http://www.lingolex.com/swom/wom-porpara.htm