"This is a good citizen."
Translation:To jest dobry obywatel.
Why is "To dobry obywatel" wrong? I think it should be accepted (I even reported that), but if I'm mistaken, please explain me. Thanks in advance!
It's impossible here, the construction with Instrumental is possible when at the beginning of the sentence (X is Y) the X is either a noun phrase or a personal pronoun (and the Y is a noun phrase, of course). "This" in "this is" serves as the subject, it is not a demonstrative creating a noun phrase.
If the sentence was "This man is a good citizen", that would be either "Ten mężczyzna to dobry obywatel" or "Ten mężczyzna jest dobrym obywatelem".