What's the difference between 'por' and 'para'?

'Por' and 'para' convey very different meanings in Spanish and using one instead of the other can change the whole meaning of the phrase. For example:-

Trabaja para Carmen - She works for Carmen (i.e. Carmen is her boss)

Trabaja por Carmen - She's working in place of Carmen (i.e. she is performing Carmen's duties because Carmen can't make it into work today)

Uses for 'por':

-Expressing the cause (not the purpose) of an action: Me caí por la nieve. I fell down because of the snow.

-Expressing movement along, through, around, by or about: Anduvimos por las calles de Jaén. We walked through the streets of Jaén.

-Expressing time or duration when something occurs. Viajamos por tres semanas. We're traveling for three weeks

-Meaning 'for' in the sense of supporting or in favor of: Trabajamos por derechos humanos. We work for human rights.

-Introducing the agent of an action after a passive verb: La novela fue escrito por Almudena Grandes. The novel was written by Almudena Grandes.

-Indicating means of transportation: Viajaré por avión. I will travel by plane.

 

Uses for 'para':

-Meaning for the purpose of or in order to: Hay que estudiar mucho para aprobar los exámenes. You have to study a lot in oder to pass the exams.

-With a noun or pronoun as object meaning for the benefit of or directed to: El regalo es para usted. The gift is for you.

-Meaning to or in the direction of when referring to a specific place: Voy para Europa. I'm heading to Europe.

-Meaning by or for when referring to a specific time: Necesito el coche para mañana. I need the car for tomorrow. Vamos a la casa de mi madre para el fin de semana. We're going to my mother's for the weekend.