Learn Finnish: Tarkkailemme piiloutuvaa jänistä.

It is formed from the verb piiloutua (to hide oneself). To make the active present participle, you take the strong verb stem piiloutu- and add the ending -va, creating piiloutuva (hiding). Because the word it describes is in the partitive case, it also gets the partitive vowel ending -a, becoming piiloutuvaa.

Why are both piiloutuvaa and jänistä in the partitive case?

The main verb tarkkailla (to observe) is a partitive verb, meaning the object you are observing must take the partitive case. Since the participle piiloutuva acts exactly like an adjective describing the hare (jänis), it must agree in case and number with the noun. Therefore, both words take partitive endings.

What is the difference between piiloutuvaa and piilottavaa?

Piiloutua is a reflexive verb, meaning the subject does the action to itself. So a piiloutuva jänis is a hare that is hiding itself. On the other hand, piilottaa means to hide something else. If you said piilottavaa jänistä, it would mean you are observing a hare that is actively hiding an object, like a carrot.

Could we express this using a relative clause instead of a participle?