I will begin with your second question: In Polish, in constructions where you would normally use 'jest' and Instrumental, it is possible to omit the 'jest' and use only 'to', where the object then stays in Nominative. This works with positive or negative statements.
-> Słoń nie jest ptakiem, kurczak jest ptakiem
-> Słoń to nie ptak, kurczak to ptak.
So 'to' is like a pointer, which gives one noun the property of being the same or from the same category as the other. (I think you can in some constructions even use both, like 'Słoń to jest bardzo dużym zwierzęciem")
The to needs to go before the negation, but I don't know why exactly. Maybe it is just the way Polish works :) I think about it as "A shirt (koszula) is/equals (to) not (nie) a dress (sukienka)"
Happy to help, dear natives I am equally happy about constructive criticisms as always :)
Well, "Słoń to jest bardzo dużym zwierzęciem" doesn't make sense, I'm afraid, you cannot use both constructions in the same sentence. Maybe you confused it with what I'm about to mention now:
But other than that, you're right. Basically I'd say you can imagine a sentence like "Słoń to (nie) ptak" to have an invisible "jest": "Słoń to (nie) (jest) ptak". Using 'jest' here (Słoń to nie jest ptak) is technically correct, but relatively rare. Anyway, this helps understanding why negation is in that place.
Good day to you Jellei, and thanks a lot for the clarification. One question still remains, could one say "Sloń to jest duze zwierzę"? Or is this only possible with the rare negated version?
Other than that, I get the rule: if "to" is used in "X to Z", don't use narzędnik/instrumental, regardless if there is "some form of być" in the sentence.
Hope I understood correctly :)