Answer:
With por, both events (or parts of a sentence) happen at the same time. The two events occuring in the same sentence are inseparable. Therefore, por usually describes the means or cause for something. It looks backwards, opening a window back in time.
With para, one event – or part of the sentence – happens after the other. It usually conveys some sort of purpose, aim or destination. That’s why it’s usually future-facing, as it acts as a gateway to upcoming events.
