This is the 225th most frequent Danish word.
hvorfor
"Why."
Used as an adverb introducing a question to inquire about reason or cause.
Hvorfor er himlen blå?
Why is the sky blue?
Used as a conjunction to introduce an indirect question about reason or cause.
Jeg ved ikke, hvorfor han kommer sent.
I don't know why he is arriving late.
Used idiomatically in a suggestion to imply that no negative reason exists for an action.