The Spanish Zone: Las postposiciones / Prepositions

Prepositions in any language are words like "at," "on," "through," "along," "to," etc., that in a sense modify the following noun. Prepositions are really important, and misusing them can result in the wrong meaning. Here are some of the most important prepositions in Spanish:

a=to, at
al=to the, at the (m.s. noun follows)
en=in
de=of, from
del=of the (m.s. noun follows)
hasta=until
con=with
conmigo=with me
contigo=with you
sin=without
*por=for, by
*para=for
sobre=on, about
encima de=on
hacia=towards
antes de=before
después de=after
durante=during
contra=against
desde=since
entre=between
cerca de=near
como=like
bajo=under
enfrente de=in front of
detrás de=behind
fuera de=outside
dentro de=inside
más alla de=beyond
según=according to
a través de=through
al lado de=beside, next to
en vez de=instead of
a causa de=because of
alrededor de=around

"Por" y "Para"
These two prepositions cause a lot of trouble for most students since they translate the same way into English, but they are not used interchangeably! You must memorize these rules, but if you practice your Spanish enough, you will get used to how they work and eventually you won't even have to think about when to use which.

Uses of "Por"

To mean "by" (Example: El libro escrito por mí, The book written by me)
To mean "per" (Example: Tomo tres comidas por día, I eat three meals per day)
To mean "through," "my means of," or "by way of" (Example: Manejamos por el barrio, We drove through the neighborhood)
To say "in the morning," "at night," etc. (Example: Lo vi ayer por la mañana, I saw him yesterday [in the] morning)
To mean "because of" (Example: Por eso, no pudimos ir a la fiesta; Because of that, we could not go to the party)
To mean "in exchange for," "instead of," or "on behalf of" (Example: Juan aceptó el regalo por su amigo, Juan accepted the gift on behalf of his friend)
To mean "for" in the sense of "in support of" (Example: Estoy por ese candidato, I am for that candidate)
In thanking someone for something (Example: Gracias por venir, Thanks for coming)
In specifying an a duration of time (Example: Duermo por siete horas, I sleep for seven hours) - Note that "para siempre," forever, is the one exception
To mean "for" in the sense of "despite being" (Example, Tienes un gran vocabulario para un joven, You have a large vocabulary for a young person)
In many idiomatic expressions!

Expressions using "por"

Por ejemplo=For example
Por supuesto=Of course
Por favor=Please
Por qué=Why
Por ahora=For now
Por lo menos=At least
Por fin=Finally
Por todas partes=Everywhere
Por lo tanto=Therefore
Por ciento=Percent
Por aquí=Around here
Por último=Finally
Por cierto=Indeed
Por allá=Around there, that way
Por lo general=In general

Uses of "para"

To mean "in order to" (Example: Necesito este curso para graduar, I need this course [in order] to graduate
To mean "intended for" (Example: Esta comida es para la cena, This food is for dinner)
To specify a deadline (Example: La tarea es para viernes, The homework is for Friday)
To mean "for" in the sense of "towards" (Example: Me voy para Madrid, I'm leaving for Madrid)

Verbs with prepositions built in
Quite a number of Spanish verbs, when translated into English, have a preposition built in, so to speak. That means when you use it in a Spanish sentence, you won't need to add the preposition as you would in English.