"I am cutting tomatoes with a metal knife."
Translation:Kroję pomidory metalowym nożem.
If you use z, it will mean more or less that you are cutting tomatoes while having a knife in your hand. Here you want to say that you are actually using the knife to cut the tomatoes, and for that you simply use the instrumental case without any preposition (that's where the name "instrumental" comes from).
In these cases, the adjectives follow the nouns, because they indicate the type, category, or class of the available item (lody czekoladowe, barszcz biały, makaron włoski).
The adjective characterizing the noun goes in front of it
(pyszne lody, gorący barszcz, wspaniały makaron).
These two rules have to be kept in mind when more than one adjective is used (pyszne lody czekoladowe, gorący
barszcz biały, wspaniały makaron włoski).
I think the english translation implies what acctually happends here. You can also cut tomatoes with a sword or something like this. Your sentense tells me WHAT are you cutting with the knife and "Kroję pomidory metalowym nożem." tells me, what is used for cutting the tomatoes!
Will the instruments always be attached to the verb and object or are they ever separated in a more complex sentence (i.e. "metalowym nożem" will always follow "kroję pomidory" and not appear somewhere else in a sentence where someone would have to search for the words)? If so, would it only be in the case where one would want to change the emphasis (e.g. With a metal knife, I cut a tomato.)?