"Yum! This bread is good."
Translation:Nam! Tämä leipä on hyvää.
I have trouble understanding why this must be a partitive. As far as I can see it is neither partial (in process or product) and it is countable (only one bread). What is the reason hyvä is partitive?
"Leipä" is used as a mass noun here, even if it's one particular sort of bread.
You could say "tämä leipä on hyvä" but I'd interpret that as you not only meaning the bread but also what's on top of it. If you have a favourite sandwich, you could recommend it to your friend by saying "tämä leipä on tosi hyvä", and they'd think it's good because it's got cheese, tomato, lettuce etc. in it, not because the bread itself is good.