Learn Finnish: Se, mikä on olennaista, tuo iloa.

In Finnish, relative clauses that act as the subject of the main sentence often need a dummy head word to attach to. Here, the demonstrative se acts as the subject for the main verb tuo. The relative clause mikä on olennaista describes that se. It literally translates to "That, which is essential, brings joy." This is how Finnish forms pseudo-cleft focus structures.

Why is the adjective olennaista in the partitive case rather than the nominative olennainen?

When an adjective follows the verb to be and describes an abstract, uncountable concept, it must take the partitive case. Here, the relative pronoun mikä refers to a general, abstract idea rather than a specific countable object, so we use the partitive olennaista.

I usually see tuo used as the word for "that". Is it acting as a demonstrative pronoun here?

No, here tuo is the main verb of the sentence. It is the third-person singular form of the verb tuoda (to bring). The core of the sentence is simply Se tuo iloa (It brings joy). Because this verb form looks exactly like the demonstrative pronoun for "that", you have to rely on syntax to recognize it as the action.

Why is iloa (joy) in the partitive case?