As an adverb, to increase grade or intensity:
?Cu?n r?pidamente caminan las malas nuevas!
How fast bad news travels! (note the accent only in questions and exclamations)
As an adverb, in relation with 'tan' and to compare:
El castigo ser? tan grande, cuan grande fue la culpa.
The punishment will be so big, as big as the crime. (translated grande as big, but 'severe' might be more suitable)
So it can mean "how" for degree or "as" when comparing.
