Learn Finnish: Väylän avauduttua jäänmurtaja jatkoi kohti pohjoisnapaa. - After the channel opened, the icebreaker continued toward the North Pole.

This is the past temporal construction. Because the action ('opening') happened before the main clause action ('continued') and has a different subject ('channel' instead of 'icebreaker'), we put the first subject in the genitive case (väylän). The verb is then formed using the passive past participle plus the partitive ending (avauduttua).

Why is there no possessive suffix on avauduttua?

In temporal constructions, you only add a possessive suffix if the subject of the action is a pronoun or if it is the exact same subject as the main clause. Because the subject here is an explicit, different noun (väylän), the verb simply ends in the partitive -a without any possessive suffix.

Why use the verb avautua instead of the more common avata (to open)?

Avautua is the intransitive version of the verb, meaning something opens by itself, opens up, or happens without a specific actor doing it. Avata is transitive and requires an actor to open an object. Since the channel just 'opened up' as a change of state, the intransitive avautua is the natural choice.

Why is pohjoisnapa (North Pole) in the partitive case (pohjoisnapaa)?