"The girl is evil."
Translation:Dziewczynka jest zła.
'Niedobra' (written together) though literally translated means 'not good', would rather be 'nasty' or 'naughty' in English if reffered to a person. As well as 'niegrzeczna' - the words can be intercheable.
When reffering to an object it would rather mean malfunction or bad taste, rather than evilness.
There may be such sentences even in this course, but I think that is technically wrong. In fact I asked some language experts about that and that's the answer that I got, that if it's just an adjective, it stays in Nominative.
Although personally I wouldn't bat an eye if I heard "Chcę być szczęśliwym", it really sounds okay to my ear.