Literally, the phrase "De quem é o cavalo?" means "Of whom is the horse?", where "de" means "of" and "quem" means "who/whom". In Portuguese, the common way to discuss ownership of an object (I say object in the grammatical sense) is to say that the object is "of the owner". So, for example, to say "Jorge's horse" you might say "O cavalo de Jorge". Thus, when you want to know to whom the horse belongs, you would ask "De quem é este cavalo?" ("Of whom is this horse?") and the response could be "Este é de Jorge".
Hope that helps.
Uma ajuda para vocês.... What= o que/ qual Who= quem Whorn= quem Where= onde When = quando Which= qual Why= por que Because= por que ( na resposta ) Whouse= de quem What time = que horas What kind of= que tipo de What color= que cor How= como How often= com que frequência How long= quanto tempo How much= quanto/ quanta ( preço ) How far= que distância How old= quantos anos
Practice makes perfect. Drill it until you have them separated. I would suggest learning them independently, so that your brain has an easier time keeping the vocabulary, spellings, etc. contextually separated. So don't do three lessons in Spanish, then three in Portuguese, then a few more in Spanish. It can get very intermingled in your head.