Learn Finnish: Korkeassa ruohossa on hiipivä susi. - There is a sneaking wolf in the tall grass.

To form the active present participle, take the strong stem of the verb and add -va or -vä (following vowel harmony rules). For hiipiä (to sneak), the stem is hiipi-, so we add -vä to get hiipivä (sneaking). It turns the action into an adjective describing the wolf.

Does the participle hiipivä change its ending like a normal adjective?

Yes! Because it acts as an adjective here, it must agree with the noun it modifies in both case and number. In this sentence, susi is in the basic nominative case, so hiipivä is too. If we said 'I see a sneaking wolf', both words would take the object ending: Näen hiipivän suden.

Could I use a relative clause with joka instead of the participle here?

Absolutely. Instead of hiipivä susi, you could say susi, joka hiipii (a wolf that is sneaking). The active present participle is simply a more compact way to express the exact same idea, and it is very common in written Finnish.

Why does the sentence put the location korkeassa ruohossa first instead of the wolf?