Learn Finnish: Paketti oli viivästynyt, koska osoite oli väärä. - The package had been delayed because the address was wrong.

The pluperfect expresses that something 'had happened'. You form it by using the past tense of the helper verb olla (here, oli) combined with the past participle of your main verb. For the verb viivästyä (to be delayed), that participle is viivästynyt.

The English translation says 'had been delayed'. Is oli viivästynyt a passive form?

No, it is active! In English, 'to delay' needs an object, so we use the passive 'had been delayed' for a package. But in Finnish, viivästyä already means 'to become delayed' or 'to run late' all by itself. Because the package is doing the action of running late, we use the active voice.

Why do we use the pluperfect (oli viivästynyt) in the first part, but only the simple past (oli) for the address?

The pluperfect shows that an action was completed before another point in the past. The package had already suffered its delay before the past moment we are focusing on. The address being wrong (oli väärä) was just a general state of things at that time, so the simple past is enough.

Could I use oli myöhästynyt instead of oli viivästynyt here?

They both translate to being late or delayed, but they are used differently. Myöhästyä is typically used when a person misses a deadline or a scheduled transport (like being late for a train). Viivästyä is used when a process or delivery takes longer than planned, which makes it the perfect choice for a package.