"It is good."
Translation:Ĝi estas bona.
Sorry, but I still do not understand. Why is "Estas bona" marked as wrong, but "Estas bone" marked as correct?
Estas bone is correct when you're talking about general statements, say you just wanted to shout out "wonderful!".
You would then say: "Mirinde!"
Thus every time it is just a general statement, like estas bone, which means something is good, not a specific it like a dog or a train, but it's just a general statement and thus -e is used.
When you're talking about a specific it like in this sentence, maybe it's a dog or a bicycle or something, then you use the adjective ending -a to describe it, as adjectives go with nouns.
Except this time "it" could be something specific in context, not a dummy pronoun. In "it is raining", it is exclusively a dummy.