Learn Finnish: Kun saavuin asemalle, tajusin, että juna oli peruuntunut. - When I arrived at the station, I realized that the train had been cancelled.

The pluperfect (pluskvamperfekti) describes an action that happened before another past action. It is formed using the past tense of the helping verb olla (here, oli) and the NUT/NYT participle of the main verb (peruuntunut).

The English translation says "had been cancelled", which is passive. Why does the Finnish sentence use the active pluperfect "oli peruuntunut"?

This uses the verb peruuntua, which means to get cancelled or to fall through. It is an intransitive verb, meaning the train does the action to itself. Even though English uses a passive construction, Finnish uses the active form of this naturally passive-like verb. If a specific person had cancelled it, you would use the transitive verb perua.

Why is "asemalle" in the allative case (onto) instead of the adessive (at)? English says "arrived at the station".

In Finnish, the verb saapua (to arrive) requires a destination case answering "where to?", not a location case answering "where at?". We use the allative ending -lle because public transport hubs like stations (asema) and stops (pysäkki) typically take outer location cases in Finnish.

I know "tajusin" comes from the verb "tajuta" (to realize). Why does it end in "-sin" here?