Can it also be "we are having (i.e. drinking) a coffee, but it is not good"?
Oh, sorry, didn't notice that.
You can say either:
Kávu máme, ale ne dobrou. = 'We have coffee, but not good.'
or Kávu máme, ale není dobrá. = 'We have coffee, but (it/the coffee) is not good.'
I try to get the point... 1. Káva máme, ale ne dobrou= We have coffee, but we don't have good coffee. HAVE COFFEE(mít káva)= DATIV, So "dobrou". 2. Káva máme, ale není dobrá= We have coffee, but the coffee IS not good. BE GOOD of something = NOMINATIVE, so ale není dobrÁ Am I right, "endless_sleeper"?
I think you mix up the cases a bit. :)
Káva máme is a non-sense. You use accusative when talking about an object of a verb and nominative when talking about a subject of a verb.
Dobrou is accusative of dobrá, which is nominative.
The verb disappears from the second clause? "but (it is) not good". Or the noun is not repeated "but not good (coffee)". With ale ne dobrou the verb has gone, no? And with ale neni dobra it is the noun "coffee" which has been omitted?
umm with "ale ne dobrou" the verb is kind of still there, it is the "ne". You could say "kávu máme, ale nemáme dobrou", but in Czech when you are using the same verb in a sentence but only in a negative form, you can just say "ne" instead. For example: "Jsme unavení, nejsme opilí." - "We are tired, we are not drunk." But you can also say both in Czech and English "Jsme unavení, ne opilí." - "We are tired, not drunk."