Learn Hardcore Finnish: Illallinen keskeytyi, kun puhelin soi. - The dinner was interrupted when the phone rang.

In Finnish, the subject of an intransitive verb takes the nominative case. Here, illallinen is the subject of the intransitive verb keskeytyä (“to be interrupted”), so it stays in the nominative. We’re saying “the dinner got interrupted,” not “we interrupted the dinner.”

Is keskeytyi a passive form? I thought passive verbs end in -t or -ttiin.

No. Keskeytyi is the 3rd person singular past tense (preterite) of the intransitive verb keskeytyä (“to be interrupted”). It’s active voice. The Finnish transitive passive of keskeyttää would be keskeytettiin (“dinner was interrupted [by someone]”), but that has a different meaning and construction.

What’s the difference between keskeytyä and keskeyttää?

Keskeyttää is transitive—“to interrupt” (someone actively does the interrupting) and takes a direct object.
Keskeytyä is intransitive—“to be interrupted” or “to come to an interruption.” It doesn’t take a direct object; its subject is the thing that gets interrupted.

Why is kun used here, and how is it different from kuin or koska?

In this sentence, kun is a subordinating conjunction meaning “when” (temporal).
• kuin is used in comparisons (e.g. “bigger than”).
• koska means “because” (causal).
Since we want “when the phone rang,” we use kun.

Do I need a comma before kun?

Yes. In standard written Finnish, you separate the main clause and the subordinate clause with a comma:
Illallinen keskeytyi, kun puhelin soi.
If the subordinate clause comes first, you also use a comma:
Kun puhelin soi, illallinen keskeytyi.

Can I invert the word order? For example, Kun puhelin soi, illallinen keskeytyi or Keskeytyi illallinen, kun puhelin soi?

Yes. Finnish has flexible word order. The two most neutral forms are:
1) Illallinen keskeytyi, kun puhelin soi.
2) Kun puhelin soi, illallinen keskeytyi.
The version Keskeytyi illallinen, kun puhelin soi is grammatically correct but sounds poetic or marked.

Why is puhelin in the nominative case and not another case?

Puhelin is the subject of the verb soida (“to ring”), and subjects in Finnish take the nominative case. If you dropped the subject entirely, you’d get the very informal Kun soi (“when it rang”), which is ambiguous, so you normally include puhelin in nominative.

Does soi mean “rings” or “rang”? How do I know which tense it is?

The 3rd person singular form soi is the same in present and past. Context tells you which one: because the main verb keskeytyi is in the past tense, soi here is understood as past (“rang”).