POR VS. PARA:
Two Spanish prepositions, 'por' and
'para',
are frequently are used for the English word 'for.'  Knowing when to use each preposition can be initially confusing.  The sections below attempt to explain when and how these prepositions should be used.


POR:

Expressing movement along, through,
around,
by, or about: 
'Anduve por las calles de la ciudad.' (I walked through
the streets of the city.) 
When used in this sense, 'por' refers to
 a general area, not a specific place. 

Denoting a time or duration when something occurs. Sometimes
is synonymous with 'durante':
'Viajamos por tres semanas.' (We're traveling for three weeks.) 

Expressing the cause (not the purpose) of an action: 
'Me
ca por la nieve'. (I fell down because of the snow.)   
'Por estudiar
mucho, acab mis tareas.' (By studying much, I finished my homework.)

Meaning per: 
'Dos por ciento.' (Two percent.) 
'Dos persons
por coche.' (Two persons in each car.)

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 John Doe.' (It was written by John Doe.)
'Fue construido
por my padre.' (It was built by my father.)

Indicating means of transportation: 
'Viajar por avin.'  (I will travel by plane.)

Used in numerous expressions: 
'Por ejemplo.' (For example.)
'Por
favor.'  (Please.)
'Por supuesto.'  (Of course)


'Por' is also used in a number of fixed expressions, such as: 
          por Dios  - for heaven's sake 
          por ejemplo  -  for example    
          por eso  -   that's why 
          por favor  -  please    
          por fin  -   finally 
          por lo general  -  generally, in general   
          por lo menos  -  at least  
          por primera/ltima vez  - for the first/last time   
          por si acaso  -  just in case  
          por supuesto!  -  of course!


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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.)
'Vive para comer.' (He
lives to eat.)

With a noun or pronoun as object, meaning for the benefit of or
 directed to: 
'Es para usted.' (It's for you.)
'La pintura
es para la casa.'  (The paint is for the house.)

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 regalo para maana.'  (I need the gift for tomorrow.)
'Vamos
a la casa de mi madre para el fin de semana.'  (We're going to my mother's
for the weekend.)