Thank you for using the Jiskha Homework Help Forum. Yes, "por" and "para" can cause problems for students. See if the following explanation will help you.
THE USES OF "PARA"
1. PARA means "for" in the sense of purpose, use, destination, for (by) a time in the future, considering that (in spite of the fact that). It is also translated "in order to, to, by."
Estudia PARA aprender (purpose) = He studies in order to learn.
Es una caja PARA dulces. (use) = It is a candy box (a box for candy).
BUT
Compr茅 una caja DE dulces. = I bought a box of candy.
Ayer sali贸 PARA Espa帽a. (destination) = Yesterday he left for Spain.
PARA el viernes estar谩 completo (future time) = For (By) Friday it will be complete.
PARA extranjero, habla bien el ingl茅s. (in spite of being) = For a foreigner, he speaks English well.
2. PARA also means PARA SER.
Estudia PARA m茅dico. = He is studying to be a doctor.
THE USES OF POR:
1. POR means "for" in the sense of "in exchange for, period or extent of time, for the sake of."
Pag贸 un d贸lar POR el cuchillo. (exchange) = He paid a dollar for the knife.
Fue al campo POR dos meses. (period) = He went to the country for two months.
Todo lo hago POR ti. (sake) = I do it all for you.
2. POR also means "for" after the verbs: ir, enviar, luchar, llamar, preguntar.
Fue (Envi贸, Llam贸, Pregunt贸) POR el m茅dico. = He went (sent, called, asked) for the doctor.
3. POR also means "during" and is often translated by "in."
POR la tarde juego al ten铆s. = In the afternoon I play tennis.
4. POR expresses the idea of rate and is equivalent to "per."
Ganan cincuenta d贸lares POR semana. = He earns fifty dollars per (a) week.
Fueron atacados POR sus enemigos. = They were attacked by their enemies.
El viajar POR avi贸n es m谩s r谩pido. = Traveling by plane is faster.
Entraron POR la ventana. = They entered through (by means of) the window.
Pasar茅 POR esa calle. = I'll pass along (through) that street.
NOTE: The following verbs are NOT followed by either POR or PARA, since the word "for" is part of the meaning of the verb.
buscar = to look FOR (to seek)
esperar = to wait FOR (to await)
pedir = to ask FOR (to request)
Buscaron un asiento. = They looked FOR a seat.
Quiero pedirle un favor. = I want to ask a favor of you. (I want to ask you FOR a favor.)
Esper贸 dos horas a su amiga. = He waited two hours for his girl friend.
Here are my two favorite examples:
a) Es un radio PARA el coche. = It's a radio FOR (that I will put into) the car. (The car gets the radio.)
b) Es un radio POR el coche. = It's a radio FOR (in exchange for) the car. (You get the radio and I get the car.)
Now when you look at your two sentences, can you see the reason why it is one or the other?
1. in order to
2. by