"The apple is not falling onto the chimney, but onto the fence."
Translation:Nem a kéményre esik az alma, hanem a kerítésre.
It's incorrect, because "esik" is negated instead of "a kéményre". If we only had the first part of Duo's sentence, your translation would be perfectly fine. But "hanem a kerítésre" makes it clear what has to be negated: the phrase that contradicts it, which is "a kéményre" here. So "nem" has to come directly before "a kéményre".