In Spanish class I learned that the right way to say "by the way" in Spanish was a propósito, but I've recently seen it translated por cierto. How exactly are these two Spanish phrases used, and are they universal through the Spanish-speaking world? Which is a more accurate way of translating "by the way" (to introduce different but related new topic of conversation)?
"Por cierto" and "a propósito" are synonymous. Both can be correctly translated as "by the way." Neither are too formal nor too slang.
As a native speaker I can say both terms are widely accepted.
See here that "por cierto" could also have the meaning of "certainly". spanish.stackexchange.com/questions/2211/… Apr 29, 2012 at 17:38
Both are widely accepted, but you could also use this other:
-Ya que estamos... (Now that we are talking about this..)
And then include a new question/comment/information/whatever related to the subject of the conversation.
They're basically synonymous but have SLIGHTLY different meanings. "A propósito" seems a bit closer to "by the way." "Por cierto" is more like "certainly," or "surely," and is a bit more emphatic. I might "translate" it as "BY THE WAY" [emphasis added].
NOTE: "Por cierto" is NOT commonly used as "certainly" in Spain, and we are still trying to determine whether or not it is used this way in parts of the "New World." But the as far as we can determine, the "authorities" permit this usage. "True" meaning of "por cierto"
"Por cierto" is never used as "surely" or "certainly" in Spain. Let's imagine I am asked "¿Vienes al cine con nosotros?" ("Are you coming to the movies with us?"). Nobody would reply with "Por cierto" to mean "Certainly".– CesarGonApr 28, 2012 at 22:19
Sorry but "Por cierto" really means (is the first meaning) of "For sure", and in other countries, like spain, takes another meaning. In latin america, because we speak a spanish more close to the antique or colonial spanish than spain, we use "Por cierto" as "De seguro". This happens because the colonies doesn't like meanings transformation like "original" country. I think the only exception for this is England but because they have another behavior for everything. The better example of this is the "vos" from "Rio de la Plata" that is a derivation of "vosotros". Jun 3, 2012 at 19:04