Questions & Answers about Puuro on lämmintä ja hyvää.
Why are lämmintä and hyvää in the partitive case instead of the basic nominative?
Because puuro (porridge) is an uncountable "mass noun" (a substance or food you can't count individually). In Finnish, when you describe a mass noun as the subject using the verb olla (to be), the adjectives following the verb must be in the partitive case. If you were describing a countable object, you would use the basic nominative form, like kuppi on lämmin (the cup is warm).
If the adjectives are in the partitive, why isn't the subject puuro in the partitive case as well?
In a basic descriptive sentence like this (A is B), the subject (puuro) stays in the basic nominative case. It's only the describing words (lämmintä, hyvää) that change to the partitive to reflect that the subject they are pointing back to is an uncountable mass.
The basic form for "warm" is lämmin. Why doesn't its partitive form use the p from its stem, like in lämpimässä (in the warm)?
Finnish words ending in -in, such as lämmin or puhelin (phone), follow a unique pattern. To form the partitive, you simply add -tä directly to the basic dictionary form: lämmin + tä = lämmintä. The stem change that introduces the p (lämpimä-) is used for almost all other grammatical cases, but the partitive skips it entirely!