"How is the bed?"
Translation:Hvordan er sengen?
I do not konw if there are definitive rules but here is my take on it.
If how is followed by an adjective or an adverb you can use hvor
how strong is he? --> hvor stærk er han?
how fast is it? --> hvor hurtigt er det?
how often does it happen --> hvor ofte sker det?
if how is followed by a verb use hvordan
how does he eat? --> hvordan spiser han?
How can he do it? --> hvordan kan han gøre det?
hvor godt? --> how good Hvordan er det godt godt? how is that good?
you can even shorten the last one down to
Hvordan godt? --> how is that good?
i hope this was of some help to you