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QUICK ANSWER
"Quién" is a form of "quién", a pronoun which is often translated as "who". "Por qué" is a form of "por qué", a phrase which is often translated as "why". Learn more about the difference between "quién" and "por qué" below.
A pronoun is a word that stands in for a noun (e.g., she).
A phrase is a group of words commonly used together (e.g., once upon a time).
A masculine noun is used with masculine articles and adjectives (e.g., el hombre guapo, el sol amarillo).
