Question

Can anyone give a simple way to know when to use por and when to use para?

Answer 1

Take a look at this article:
http://www.studyspanish.com/lessons/porpara.htm

There are many, many uses for POR and PARA.

But since they both mean "for,"
I'll give you a quick run-through.

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POR is used, to say "for," when...

- There is an action being done, for someone else.
"Yo lo hago por ti."
"I do it for you."

- We are expressing a duration, or length of time.
"Yo estudié por cinco horas."
"I studied for five hours."

- We are expressing thanks, or apologies.
"Gracias por la invitación."
"Thank you for the invitation."

- We are talking about a trade, or exchange.
"Por el libro, yo te pagaría cinco dólares."
"For the book, I would pay you five dollars."

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On the other hand,
PARA is used, to say "for," when....

- We are expressing a location, or destination.
"Yo salí por tu casa."
"I left for you house."

- We are talking about an item that is for someone or something else.
"El libro es para ti."
"The book is for you."

~~~~~~~~~~~~~

These aren't ALL the uses of POR and PARA,
but they are the basic differences, whenever we're trying to say "for."

~~~~~~~~~~~

There IS a tiny exception, for when we want to say "forever."

I did tell you that when we are expressing a duration of time,
we usually use POR.

But when we want to say "forever,"
we actually say PARA SIEMPRE.

It's a bit of an anomaly.