How do I know whether to use por or para?

Because these words are both translated to English with 'for' it can be difficult to choose between them at times. In order to choose correctly, there are several rules to remember, here are the most important ones:

Por

1) For a duration of time:

Estudié por dos horas - I studied for two hours

2) The reason for something:

Estoy cansado por falta de sueño - I am tired because of a lack of sleep

3) Frequency

Voy al cine dos veces por semana - I go to the cinema two times a week

4) In passive sentences

El edificio fue construido por Juan - The building was constructed by Juan

5) To thank someone for something

Gracias por la ayuda - Thank you for the help

Para

1) Purpose of an action/goals

Estudio para aprobar los examenes - I'm studying in order to pass the exams

2) Recipient 

Este regalo es para Maria - This present is for Maria

3) Deadlines

Los deberes son para el viernes - The homework is for Friday

4) Physical destinations

Voy para Chile mañana - I am going to Chile tomorrow

(Note you can also say 'Voy a Chile mañana' here)

There are more situations that require 'por' and 'para' but these are some of the most important ones. To help you remember, think that 'por' generally looks backwards to the reason why something happened, and 'para' generally looks forwards towards the goal or purpose of an action. However, the only way to master these is to memorise the rules and different situations in which you use 'por' and 'para'.