Hi Adelantino! "ένα ποτήρι κρασιού" talks about the type of glass: "a wine glass"! Sometimes "of" takes the meaning of "με" (with). Properly, "a glass of wine" would be "ένα ποτήρι με κρασί" (a glass with wine), but graecophones commonly just say "ένα ποτήρι κρασί". It's the same with cups (ένα φλυτζάνι τσάι), plates (ένα πιάτο ρύζι), bottles (ένα μπουκάλι νερό), etc... Keep it up ;-)
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As for κρασί, it is (nom.) το κρασί, (gen.) του κρασιού, (acc.) το κρασί, (voc.) - κρασί . Wiktionary is usually good at giving you accurate results even when typing a declension/conjugation form in the search box. Also check out the resources links put together by the Greek team in the Greek forum: https://forum.duolingo.com/topic/936.
In the exercise, the accusative is implied: ένα ποτήρι (με) κρασί. Prepositions are followed by the accusative. Three out of four cases are identical for all neuter words.