"The mämmi is black and sweet."
Translation:Mämmi on mustaa ja makeaa.
Is it really correct that it's "The mämmi"? Because if it's referring to maybe a bowl of mämmi on the table, it shouldn't the partitive for the adjectives as well, right? In my understanding it should be "Mämmi on makea" for "The mämmi is sweet" and "Mämmiä on makeaa" for "Mämmi is sweet". Please let me know what I'm missing here.
All Finnish nominals are subject to inflection. If what's confusing you is not the fact that adjectives are being inflected, then I'm guessing you're wondering why partitive case in particular. In that case, it's because "mämmi" is a mass noun and as such triggers partitive case for the adjectives that modify it, since indefinite amounts trigger partitive case.
You could use those nominative forms when they precede the noun they modify. Perhaps Musta ja makea mämmi on hyvää, "The black and sweet mämmi is good".
You can also use the nominative forms when they're the predicative of a countable noun. Marja on musta ja makea, "The berry is black and sweet".