"These are not tomatoes."
Translation:To nie są pomidory.
in sentences like this is. we always use to (or tamto =that)
also in sentences like Kasia is a girl we have Kasia to (jest) dziewczyna to doesn't change
Just trying to get my head around the use of nominative and genitive with negative sentences: tomatoes stays nominative here as the sentence is just a simple declaration(?) but if the tomatoes are the subject of a verb in the negative then they would be pomidorów?
It's actually easier. You have to look at how the 'positive' sentence would look like. If the positive sentence took Accusative, it turns into Genitive. Other cases just stay the same when negated.
Any sentence starting with "This/That is", "These/Those are", no matter the grammatical gender or number, will start with "To" which then serves as the subject of the sentence and not any kind of a determiner.
And if anything, "These" would be translated as "Te". "These tomatoes" = "Te pomidory".
That's the very basic word order. The only difference from English is the placement of the negation. But you negate the whole notion of 'being tomatoes', not just tomatoes. Otherwise you'd end up at something like "These are not-tomatoes".