La granda malbona lupo!
kurus rapide!
Knock knock
kiu estas tie?
Why is "what is there" considered wrong?
Because "Kiu?" is "Who?"
(Or it could be "Which one?". But not "What?" -- that is "Kio?".)
"Kiu" means "who" or "which one", as in "Kiun skribilon vi prenis el mia skribilulo?"
How would you say who is at the door would you say kiu estas ĉe la pordo ?