"Does he know the answer?"
Translation:Он знает ответ?
In one example duo said: я не знаю ответА but when i entered он не знает ответА it was not accepted,, i believe that in both cases the word ответ should remain in the nominative because its inanimate.. So which one is the right answer??
You're referring to the formation of accusative. For animate masc. and neuter nouns, it's same as genitive. For inanimate masc. and neuter nouns, it's same as nominative. That's true.
For, in negative sentences you can also put direct object in genitive (i.e. not accusative looking like genitive, but genitive). The rules for choosing when genitive is applicable and when it's not are somewhat obscure, but «я не знаю́ отве́та» and «он не зна́ет отве́та» both sound OK to me. IMHO both should be accepted.
Yes, you're right.