Step 1

Por and para both mean for.
However, they are not interchangeable.

Use por to indicate reason or motive:

Estoy aquí por asuntos de negocios.
       I am here because of (for) business reasons.

Use por when discussing time or day:

Hago la tarea por las noches.
       I do homework at night (for nights).

Use por when expressing exchange:

Te doy mis palabras por tu dinero.
       I give you my words for your money.

Use para to indicate purpose or use :

Sonia va al supermercado para comprar el pan.
       Sonia goes to the grocery store in order (for)
        to buy the bread.

Use para to express destination: recipient (a person):

María va a comprar un regalo para él.
       Mary is going to buy a gift for him.

Use para to express destination: location (a place):

¿A qué hora parte un tren para Berlín?
       What time does a train leave for Berlin?