The tips explain the difference in usage (https://www.duolingo.com/skill/el/Quest ... -and-notes), and it seemed quite familiar to me because at least partly it is similar in Spanish:
- to ask e.g. for a name, an address, a phone number and so on, you need cuál instead of qué: ¿cuál es tu nombre/dirección/número de teléfono?
- You would always use qué before nouns at least in Spain: ¿qué trabajo haces?
However, in Spanish you can use ¿qué es ...? in order to ask for a definition: ¿qué es una jirafa? Is it the same in Greek, and τι είναι καμηλοπάρδαλη; would be correct in this case? (It is what Google translate suggests, but I've been taught not to trust it too much.)
And then, to express a question like which cat is yours?, I'd probably use either ¿qué gato es el tuyo? or Spanish, or change the sentence to ¿cuál de los gatos es el tuyo? (which of the cats is yours?), but would avoid ¿cuál gato es el tuyo? though it may be said this way in Latin America.
Would all of these work in Greek, too? (Google translate literally here, so ¿cuál gato es el tuyo? becomes ποια γάτα είναι η δική σου;)