Difference between “por” and “para”

Question

Even after taking 4 years of college Spanish and living abroad, I still don't have a very firm control of when to use por or para. What are the basic rules on when to use either.

Answer 1

From here (with minor corrections by me):

Uses for por:

Expressing movement along, through, around, by or about:

Anduve por las calles de la ciudad.

I walked through the streets of the city.

Denoting a time or duration when something occurs.

Viajamos por tres semanas.

We're traveling for three weeks.

Expressing the cause (not the purpose) of an action:

Me caí por la nieve.

I fell down because of the snow.

Meaning per:

Dos por ciento.

Two percent.

Meaning supporting or in favor of:

Trabajamos por los derechos humanos.

We work for human rights.

Introducing the agent of an action after a passive verb:

Fue escrito por Bob Woodward.

It was written by Bob Woodward.

Indicating means of transportation:

Viajaré por avión.

I will travel by plane.

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, se 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 usted.

It's for you.

Meaning to or in the direction of when referring to a specific place:

Voy para Europa.

I'm heading to Europe.

Answer 2

para indicates direction or intention

por indicates origin or cause

A Spanish teacher explained the difference with an "intention arrow". In the case of 'para', the intention moves from the subject to the direct object:

Las flores son para ti.     flores -> ti
Yo trabajo para ti.         yo -> ti          [I work for/to please you]

In the case of 'por', the intention moves from the direct object to the subject. This requires a little more imagination and liberal interpretation of "intention."

Yo trabajo por ti.          yo <- ti          [I work in your place]
Yo voy por camión.          yo <- camión
Yo ando por la ciudad.      yo <- ciudad
Tu Me llamas por teléfono.  tu <- me
Él pasa por mi.             él <- mi

Answer 3

I know this is old but I'd like to add my answer as well. The lists have been taken from a grammar textbook but most examples (except the last two) are not just pasted... sometimes I needed some help with the structure or ideas, though. :) So if you spot a mistake, feel free to correct.

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Por

1. Causa (cause)

   Todo sucedió por ti. — Everything happened because of you.

2. Complemento de agente de la voz pasiva (it indicates the complement in the passive)

   La carta fue escrita por Juan. — The letter was written by Juan

3. Localización aproximada (rough localization)

   La casa estaba por aquí. — The house was around here.

4. Movimiento a través de un lugar (movement through a place)

   El hombre pasea por el parque. — The man goes for a walk in the park.

5. Periodicidad (regular recurrence)

   Voy a las clases cinco veces por semana. — I attend the classes five times a week.

6. Época aproximada, pasada o futura (rough indication of time, past or future)

   Por esa época, casi nadie tenía zapatos. — Around that time, almost nobody had shoes.

7. Sustitución (substitution)

   He llamado al representante por ti. — I called the spokesman in your place.

8. Medio (medium)

   Llámame por telefono. — Call me on the phone. (via phone)

9. Precio, definido o indefinido (price, definite or indefinite)

   He comprado un bocadillo por dos euros. — I bought a sandwich for two euros.

Para

1. Finalidad (finality, purpose)

   He venido aquí para ayudarte. — I came here in order to help you.

2. Destinatario (addressee)

   He llevado algo para ti. — I brought something for you.

3. Opinión

   Para mí, debería irse. — In my opinion, he should go away.

4. Limite espacial, destino (spatial limit)

   Me fue para mi casa. — I went home.

5. Plazo, fecha limite (place or date that represents a limit)

   Él dicho que acababa de interrogar para el viernes. — He said he was going to stop examining (the students) by Friday.

6. No estar para (ausencia de disposición anímica)

   Hoy no estoy para nadie. — Today I'm not available for anybody.

7. Estar para (expresses an imminent action)

   Estaba para llamarte. — I was about to call you.

8. Valor concesivo (to express something like "Even if...")

   Para tener esa edad, es muy hermosa. — Even if she is that/considering her age, she's very beautiful.

9. Para + sustantivo —> comparación y valoración (positiva o negativa);

   Para playas sucias, las de esta zona. (not sure about translation)

10. Para + oración —> comparación y valoración (negativa o irónica).

    Para lo que has dicho, mejor habría sido que te callaras. — Considering what you said, it would have been better if you had shut up.

Answer 4

To put the Por or Para question into something easier to remember, I have put the reasons (taken from Puntos de Parttida Spanish Textbook) in a visual style similar to Flimzy's arrows of intention, but with a few additions on the Por side.

Por is represented by a backwards arrow to represent the following meanings:

         Because of,
         Due to

Por is represented by a circle with an arrow running through it for the following meanings:

         Duration, (length of time pictured by the part of the arrow inside the circle)
         By means of,
         Through

Por is represented by a recycle symbol for the following meanings:

          In exchange for,
          In place of

Para is represented by a forward pointing arrow:

        In order to, 
        To be used for,
        To be given to,
         Future deadline,
        In the employ of,
        Towards

(One meaning of para that doesn't fit with the forward arrow is "in comparison with others in the group." I don't have a picture for that one.

If anyone knows how to put visuals in here to help with this explanation, please let me know. It would look better with the circles and arrows actually drawn in. I'm very new to this.