We said a few things about por and para a few days ago. Most of the times, explanations about these prepositions look nice and clear but it all becomes confusing when you start using them in conversation.
Out of all of the uses these prepositions have, there are two that probably are the most confusing ones:
Para expresses a purpose, while por a cause, a reason.
Let’s see a two examples:

• Llamaré por teléfono a Carlos ______ pedirle mis cosas.
• ______ levantarte tarde, perdiste el tren.

Which sentence expresses a purpose? Which one a cause? “pedirle mis cosas” is the purpose of the call. Getting up late made me (caused) lose the train.
Now, let´s think of them logically, in terms of time: Temporally, the purpose (not the enunciation of the purpose but its actual fulfillment) comes always after the main action. First, I call Carlos, after that, I will ask for my things. Here, we will always use PARA.
The cause always precedes the main action. I woke up late and only then, for that reason, I lost my train. In this case we will use POR.
Voy a Chile para estudiar español. I go to Chile to learn Spanish. I first go there then, I will study Spanish. The purpose will actually take place after I arrive in Chile.
Estoy triste por lo que le pasó a Miguel. I feel sad for what happened to Miguel. Temporally, something happened to him first and then I feel bad. But the cause takes place first.