Have we discussed nouns modifying other nouns yet? Is the rule here just to use the appropriate form of ka? E.g.:
work clothes = kam ke vastr
butter knife = makkhan ka chaakoo
Can someone explain to me why we use उसे here and not वह?
चाहिए is always used with the dative form of a pronoun (उसे, मुझे, हमें etc) or a noun+को.
Think of it as being literally translated as 'To her, fever medicine is needed'.
I wrote उसको बुखार की दवाई चाहिए and it wasn't accepted. Isn't it the same?
It should have been accepted. You can report if you see the sentence again.