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QUICK ANSWER
"What" is a form of "what", a pronoun which is often translated as "qué". "Who" is a pronoun which is often translated as "quién". Learn more about the difference between "who" and "what" below.
who(
hu
)A pronoun is a word that stands in for a noun (e.g., she).
a. quién
Hello? Who is calling?¿Hola? ¿Quién llama?
I don't know who he is, but he looks a lot like my brother.No sé quién es, pero se parece mucho a mi hermano.
2. (relative)
what(
wuht
)A pronoun is a word that stands in for a noun (e.g., she).
An adjective is a word that describes a noun (e.g., the big dog).
An interjection is a short utterance that expresses emotion, hesitation, or protest (e.g., Wow!).