Learn Finnish: Jos olet hiljaa, voit nähdä hiipivän suden. - If you are quiet, you can see a sneaking wolf.

You take the strong vowel stem of the verb, which is hiipi-, and add the active present participle ending -vä, giving you hiipivä (sneaking). Because participles act like adjectives, it can then take case endings, which is why it has the -n here.

Why do both "hiipivän" and "suden" end in "-n" here?

They make up the total object of the sentence. When you see a whole, complete thing (the sneaking wolf), the singular object takes the accusative case, which looks exactly like the genitive -n. Because participles agree with the noun they modify, just like adjectives, hiipivä must match suden, so both take the -n ending.

Why does this sentence use "hiljaa" instead of the adjective "hiljainen" for "quiet"?

Hiljaa is an adverb meaning "silently" or "without making noise." If you used the adjective hiljainen (olet hiljainen), it would mean you are generally a quiet person by nature. The sentence is about being quiet right now in this specific moment, so the adverb is used.

What happens to the word "susi" (wolf) to make it "suden"?