"Are you eating using a knife?"
Translation:Spiser du ved hjelp av en kniv?
"spiser du av en kniv?" = "are you eating off (of) a knife?" which means something else, but "spiser du med en kniv"? (med = with) means roughly the same as the original sentence. "Spiser du ved hjelp av en kniv?" could imply that you would not be able to eat unless you were using a knife, so there is a subtle difference.