Why is not 'which' drug . . . and why in some of these sentences is 'drug' and acceptable translation for lægemiddel but not for others?
You're right, the English translation should be which.
Yeah, how do we know if we should use hvad or hvilke-t-n ? :/
That's easy. Don't use hvad before a noun.
When do little Danish kids learn all these rules and big words??
Like all other native speakers of any language - at a young age through magic.
By 'magic', you probably mean 'intense practice and many mistakes'.
Duos prompt said for not imod was choice number 1. ????
English does it kinda wrong here, so the translation hints are a bit off. You don't take medicine "for" (supporting) pain, but "against" it. Imod means "against".