The problem is that two Spanish prepositions, por and para, frequently are used for the English word "for. In Spanish, the key to understanding which preposition to use is to think of the meaning you want to convey. If I use a phrase such as "three for a dollar" in English, the "for" has a different meaning than it does in "this book is for you." In the first case, "for" indicates an exchange or a rate, while in the second case it indicates an intention or direction. Thus the Spanish translation of the two phrases are different, "tres por un dólar" and "este libro es para ti."
Major uses of these two prepositions:
Uses for por:
Expressing movement along, through, around, by or about: Camine por las calles de Madrid. I walked through the streets of Madrid.
Denoting a time or duration when something occurs. Estaremos fuera por tres meses. We will be out for three months.
Expressing the cause (not the purpose) of an action: Me resbale por el hielo. I slipped because of the ice.
Meaning per: Cien por ciento. Hundred percent.
Meaning supporting or in favor of: Trabajamos por derechos humanos. We work for human rights.
Introducing the agent of an action after a passive verb: Fue escrito por Raymon Perez. It was written by Raymon Perez.
Indicating means of transportation: Viajaré por barco. I will travel by boat.
Used in numerous expressions: Por ejemplo. For example. Por favor. Please.
Uses for para:
Meaning for the purpose of or in order to: Para bailar la bamba, necesita una poca de gracia. In order to dance the bamba you need a little grace.
With a noun or pronoun as object, meaning for the benefit of or directed to: Es para ti. It's for you.
Meaning to or in the direction of when referring to a specific place: Voy para Miami . I'm going to Miami.
Meaning by or for when referring to a specific time: Necesito el carro para mañana. I need the car for tomorrow.