"Is this a pulla or bread?"
Translation:Onko tämä pulla vai leipä?
So do I understand correctly...:
It is because pulla and leipä are nominative, not partitive, that I know the speaker is speaking about one specific bun or loaf.
Were the speaker to use partitive, I would know they are speaking more generally of a range of baked goods they are asking about.
The way I understand it is that if you're referring to a discrete object then it gets nominative, and in this Duolingo course it would be translated into English with "a", as "a pulla". But if you're talking about the substance an object or objects are made of, then it gets the partitive and would get translated into English as just "pulla", not "a pulla".
I imagine stuff like this makes more sense in context, but the sentences given here don't provide any context, and so I think that's why they rely on using specific English translations for each case, even when the English sentence could be interpreted in a number of ways. I mentally try to fill the gaps by thinking, ok, if they phrase it this certain way in English, they're probably looking for this particular case in Finnish, and vice versa.