"Whose horse is it?"
Translation:De quem é o cavalo?
is there something wrong with this word order, "o cavalo é de quem?" i thought it would work as well...
Nothing wrong with the order when asking that question. :) But maybe since the order wasn't like that in English, they won't accept it. Did you click on "report a problem" to see what they think?
No. "Cujo" is not used in the beginning of a sentence, only between the two terms it refers to (O homem cujo cavalo... / The man whose horse...).