Questions & Answers about Piha on märkä, joten en juokse siellä tänään.
Because piha is the subject of the clause, so it’s in the nominative: piha = the yard.
Pihan is the genitive (“of the yard / the yard’s”), and would need a different structure, e.g. Pihan pinta on märkä (The yard’s surface is wet).
It is matching: in Finnish, a predicate adjective agrees with the subject in number and case.
Here the subject is piha (singular nominative), so the adjective is also märkä (singular nominative).
Compare:
- Pihat ovat märkiä. (The yards are wet. → plural partitive predicate adjective is common)
- Piha oli märkä. (The yard was wet.)