So is "dobre" the singular, neuter, nominative form of "good" ?
Yes
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 ;)
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 is the translation for "This apple is good"?
To jabłko jest dobre
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"
Jestem z Polski =I am Poland. Art = plastyka A te pytanie było banalne
I am Poland = Jestem Polską. Art = sztuka.
*A te to pytanie było banalne.
A maçã é boa
"This apple is good" doesn't work ://///////
"this" is your own addition to the sentence.