Translation:He has used a knife.
I bet Terence Trent D'Arby didn't expect that when he asked him to sign his name.
Would it not be 'on użył nóż'?
"używać" (kogo? czego?) takes Genitive.
I think here in this sentence ,the verb is "użyć" instead of "używać".
True. But that doesn't change the case :)