Questions & Answers about Märkä hiekka on painavaa.
Why is painavaa in the partitive case, while märkä is in the nominative case?
Hiekka (sand) is a mass noun, meaning it is an uncountable substance. In Finnish, when the subject is a mass noun, the adjective describing it after the verb olla (is) must be in the partitive case. That gives us painavaa. However, märkä comes directly before the noun it describes as an attribute, so it must match the noun's case, which is the basic nominative.
Would it be wrong to say "Märkä hiekka on painava"?
Yes, it would sound unnatural. Using the nominative painava makes it sound like "Wet sand is a heavy individual object." Because sand is a continuous substance rather than a single countable item, you need the partitive painavaa to express that it is heavy stuff.
Why isn't the subject itself in the partitive? Couldn't we say "Märkää hiekkaa on painavaa"?
No, because this is a direct descriptive sentence ("A is B"). In this structure, the subject is almost always in the nominative case (märkä hiekka), even when it is a mass noun. If you put the subject in the partitive (märkää hiekkaa on...), it changes the grammar to an existential sentence meaning "There is some wet sand...", which requires a location rather than a descriptive adjective at the end.
How is the partitive formed for the word painavaa?