"It is the cook's knife."
Translation:Det är kockens kniv.
You could if you’re pointing and say ”that one” - ”that’s the cook’s knife”. But in this sentence, the ”it” does not refer to the ”knife”. It’s just a formal pronoun because the sentence needs some sort of subject. It’s similar to English ”it is raining”. The ”it” does not really refer to anything in particular there either.
Kockarna = The cooks (two, or more cooks) Kocken = The cook (one single cook)
The 's' added to the words above mean that they (the cook, or the cooks) possess the object (in this example several knives):
Kockarnas knivar = The cooks' knives. (Here you talk about the knives that several cooks have, or share. In English indicated with the ' after the plural s.)
Kockens knivar = The cook's knives. (Here you talk about the knives that one cook has. This is indicated with a 's in English.)
I hope it helps you. :) If not, feel free to ask for clarification. :)