So is "dobre" the singular, neuter, nominative form of "good" ?
K, I'm russian, and "dobre" on russian sounds like "kind", and just imagine: kind apple
"kind" can be also a translation of Polish "dobry"... where it makes sense, of course ;)
Shouldnt it be Jablko jest dobrym? since after jestem, jestesz etc its instrumental case?
No. In the "noun is adjective" type of sentence, you use adjective in nominative, and you can't use "to"
What's the difference between dobrze and dobre?
Dobrze is the adverb, dobre is the neuter singular adjective.
Most adverbs are formed by adding -ie to the stem, and this is also the case here.
You just have to apply the phonological rule r + i -> rz.
What's the question? That's not how it's written.