"The soup is good."
Translation:Zupa jest dobra.
Why is "Zupa to dobra" incorrect? Can "to" be used only with 2 nouns? Is "Zupa to jedzenie" gramatically correct?
And do I use "to" as-is or change it to match the noun? For example, what's correct: "Zupa to jedzenie" or "Zupa ta jedzenie"?
"to" doesn't change in such a construction. And yes, it can be used only when both parts are noun phrases.
No, it's clearly feminine in Polish. Almost all nouns ending with -a are feminine.
There's no such a form as "dobrę" at all, in any case.
This is a simple sentence using only Nominative, "soup" is feminine, so it's "dobra".