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QUICK ANSWER
"Quién" is a form of "quién", a pronoun which is often translated as "who". "A quién" is a phrase which is often translated as "to whom". Learn more about the difference between "a quién" and "quién" below.
a quién()
A phrase is a group of words commonly used together (e.g., once upon a time).
a. to whom
Si no sabe a quién dirigir la carta, llame y pregunte.If you don't know to whom you should address the letter, inquire about it over the phone.
quién()
A pronoun is a word that stands in for a noun (e.g., she).
