"This is a good citizen."

Translation: To jest dobry obywatel.

[chico.rios]

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!

[Gerardd88]

It's correct. Have you reported it?

[chico.rios]

Yes, I've reported it, fortunately!

[random8042]

what is the construction with the instrumental case?

[Jellei]

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".

[Ollyfer]

This would actually make a good portemanteau: »To jest dobrywatel.« ;-)