"Co to jest szczęście?" was rejected. Is there a subtle difference between this and "czym jest szczęście"?
Perhaps a subtle ('czym' sounds more philosophical to me, yours a bit like asking for a definition), but your answer should surely be accepted. Added now.
If anything, then "jakie" ("szczęście" is neuter), but you only use those when "what" means basically the same as "what kind of", or when you ask for a description.
czym instrumentalis ?
Dlaczego nie Co jest szczęście?
Imagine the answer: "Szczęście jest uczuciem" (Happiness is a feeling). The noun "uczucie" takes Instrumental in such a sentence, which makes it easier to see why also "co" needs to take Instrumental in the question.
Thanks Jellei. So if I ask Co jest szczęście? the answer would be Opanowanie jest szczęściem?, or is this a fallacy?
Opanowanie? Composure? Interesting choice...
Anyway, "Co jest szczęście?" grammatically is wrong. To get the answer that you presented here, the question should be "Co jest szczęściem?".
the translation my dictionnary showed was "serenity". Maybe that´s wrong...
Dziękuję za wyjaśnieniem!
I'd say "spokój", but perhaps in some way it works... I'd need a further context.
Nie ma za co! :) But a small correction, if I may: "Dziękuję za wyjaśnienie!" (for thanking for something, it's "za" + Accusative).