Para is used to express: use or destination (for), purpose (in order to); point of future time (for, by) and to be about to (estar para + infinitive – though this is not used in all dialects of Spanish.)
La carta es para Concha. The letter is for Concha.
Estudia para aprender. He studies in order to learn.
Lo tendré para el martes. I will have it by Tuesday.
Juan está para salir. John is about to leave. (not used in all dialects)
Por is used to express: a place through or along which; expressions of time (in, during, at); exchange, price (for); unit of measure (by, per); way or means (by); because of, on account of, for; to go for, to send for; on behalf of, for the sake of; motive, reason. It is also used after a passive verb to indicate the agent (by) and estar por + infinitive indicates what remains to be done or to be in favor of.
por el pueblo through the town
por la mañana in the morning
Pagó un peso por el libro. He paid a dollar for the book.
Se vende por libras. It’s sold by the pound.
Voy por tren. I’m going by train.
Voy por Alicia. I’m going for Alice.
Voté por Juanita. I voted for Juanita.
Fue escrito por Cervantes. It was written by Cervantes.
La carta está por escribir. The letter is yet to be written.
Estoy por escribirla. I am in favor of writing it.
Pero (but) usually follows an affirmative expression, but may follow a negative statement if the verb of the first clause is repeated, or if another verb follows.
Bebe leche pero no bebe café. He drinks milk, but he does not drink coffee.
Sino (but) is only used in negative sentences of contrasting statements when the verb of the first clause is understood but not repeated.
No bebe café sino leche. He does not drink coffee, but milk.
Links
http://www.ielanguages.com/spanish3.html
http://www.youtube.com/watch?v=_cBJoq479NM
Next: 061 Spanish Object Pronouns
Word Document at: 060 Spanish Para vs. Por and Pero vs. Sino
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Dr. Hugh Fox III 