"That soda pop is black."
Translation:Tuo limonadi on mustaa.
In most sentences, the subject is nominative, even if it's an uncountable mass noun. However, the adjective following on will be partitive if the subject's uncountable, but nominative if the subject's countable and singular.
I think the only partitive subjects used in this course are in existential and possessive sentences, where the sentences start with either the location or the possessor. Thus Täällä on limonadia and Minulla on limonadia.