i accidentally hit caps lock before typing out my translation, so it made me look like an angry serial killer xDDD
"potrzebować" takes Genitive. If it helps, you can think of it as "to have a need of".
Thanks! That's a great tip on how to remember that "need" takes genitive. I was thinking of it more like have or the other verbs that take accusative.
I thought previously we had 'inny' as meaning 'other', so I thought this meant 'I need the other knife'. How should I say that, please?
As 'the other' probably means "not this one, the second one", I'd go with "Potrzebuję tego drugiego noża" (kinda 'this second one').
Well, as usual, it's about cases. And genders. Firstly, you have to understand that "nóż" is masculine (like most nouns ending in a consonant). So it needs a masculine adjective: "inny nóż".
Now, "potrzebować" takes Genitive. The Genitive forms of "inny" and "nóż" are "innego" and "noża" respectively.
"inne" would be Nominative or Accusative for neuter singular or 'not masculine-personal plural'.