I have a preposition for you: using por and para in context.

First, let's review same basic grammar.  A preposition is a word that shows purpose, destination, location, direction, or time.  Prepositions are words such as to, for, by, through, with, from, and in.  Prepositions describe more fully the destination, direction, object, recipient, purpose, or cause of an action verb. 

I walked into the room.  Into is a preposition telling you where I walked. 
I gave a book to Marie.  To is the preposition showing the recipient of the gift. 

Two Spanish prepositions that are commonly used (and frequently confused) are por and para.  Para means for or to, and por means by, through, or for.

Para is used when you have a target for your verb, such as a person, date, event, purpose, reason, or object.  Para can be used to indicate a recipient, to express a deadline, to indicate destination, or to show purpose:

Este regalo es para mi padre.  (This gift is for my father.)
Necesito mi tarea para el martes.  (I need my homework by Tuesday.)
Mi mamá salió para Nueva York.  (My mother left for New York.)
El vaso es para agua.  (The glass is for water.)

Por is used when you want to thank someone because he or she has helped you.  In some instances, por is used to indicate frequency or to express a length of time, and in a mathematical context, it is used for multiplication.

Gracias por tu ayuda.  (Thank you for your help.)
Voy a clases 5 veces por semana.  (I go to class 5 times per week.)
Yo estudié por dos horas.  (I studied for two hours.)
Dos por dos son cuatro.  (2 x 2 = 4)