Por versus para = for vs in order to vs to

Por versus para Por and para are both prepositions, which means that they must be followed by a prepositional object (a noun, pronoun or infinitive). Both can sometimes mean for in English, but they can also be translated in many other ways depending on the context, because they mean different things. Para tends to focus on a destination of some type. Por focuses more on the means to an end or the intervening step or duration. Here are some samples of their uses:
BY, BY MEANS OF		(IN ORDER) TO
Van por tren. They go by train.		Viven para comer. They live to eat.
		Estudio para médico. I'm studying to be a doctor.
THROUGH, ALONG		FOR (toward, in the direction of)
Voy por el parque. I go through the park.		Salen para Madrid. They're leaving for Madrid.
DURING, IN (the morning, etc.)		FOR, BY (a specified future time)
Leo por la tarde. I read in the afternoon.		Lo leo para el lunes. I'll read it by Monday.
BECAUSE OF		FOR (compared with others)
Estoy triste por el accidente. I'm sad because of the accident.		Es alto para su edad. He's tall for his age.
(IN EXCHANGE) FOR		(DESTINED) FOR, (to be given to)
¿60 pesos por eso? 60 pesos for that?		El regalo es para ti. The gift's for you.
Gracias por el regalo. Thanks for the gift.
FOR (+ a period of time)
Comió por 3 horas. He/She ate for 3 hours.
FOR (THE SAKE OF), ON BEHALF OF		FOR (in the employment of)
Lo hago por ella. I do it for her.		Trabajo para ella. I work for her.
FOR (in order to get, in search of)
Voy por el libro. I'm going for the book. fuente :

http://users.ipfw.edu/jehle/COURSES/porpara.htm