Learn Finnish: Laulava pöllö istuu puussa. - The singing owl is sitting in the tree.

Laulava is the active present participle, which turns a verb into an adjective meaning "doing [verb]". To form it, take the strong stem of the verb (here, laula-) and add -va or -vä depending on vowel harmony. So, laulaa (to sing) becomes laulava (singing).

Does laulava have to match the case of pöllö?

Yes! Because the present participle acts just like a regular adjective, it must agree with the noun it describes. Since pöllö is in the basic nominative case here, laulava is too. If you wanted to say "I see the singing owl", both words would take the partitive case: Näen laulavaa pöllöä.

How would I say "The owl singing in the tree..." using this participle?

In English, we can put "singing in the tree" after the noun. In Finnish, participle phrases must go before the noun they describe. To say "The owl singing in the tree", you have to package it as "The in-the-tree singing owl": Puussa laulava pöllö... In our current sentence, however, laulava just stands alone describing the owl, and istuu puussa (is sitting in the tree) is the main action.

What is the difference between saying laulava pöllö and pöllö, joka laulaa?