Questions & Answers about Tuo virtaava joki on kylmä.
How do we form the participle virtaava from the verb virrata?
To form the active present participle, use the third-person plural (he) form of the verb in the present tense. For the type 4 verb virrata (to flow), that form is virtaavat. Drop the final -t, and you get virtaava. Notice how this brings out the strong consonant grade (rt) compared to the weak grade in the infinitive (rr).
Why use a participle here instead of a relative clause like joka virtaa?
You can use either! Tuo joki, joka virtaa, on kylmä (That river, which flows, is cold) is grammatically correct. However, using the -va/-vä participle allows you to pack the verb's action into an adjective. This makes the sentence much more concise, and is a very common structure in Finnish.
If I wanted to say "in that flowing river," how does the participle change?
The active present participle behaves exactly like a normal adjective, meaning it must agree in case and number with the noun it modifies. To say "in that flowing river," you put all the words in the inessive case: tuossa virtaavassa joessa. (As a quick refresher, remember that joki undergoes consonant gradation and changes to the stem joe- before adding the ending!).
I sometimes mix up the pronouns tuo and se. Why is tuo used in this sentence?