Learn Finnish: Katsoin merta, kunnes tuuli tyyntyi. - I watched the sea until the wind calmed down.

Kunnes acts like most standard subordinate conjunctions (such as että or koska). It starts the subordinate clause, and standard subject-verb word order follows it. In this sentence, we have the subject tuuli right before the verb tyyntyi.

Does kunnes force both clauses to be in the same tense?

Not necessarily, but they must make logical sense together. In this sentence, watching the sea and the wind calming down both happened in the past, so both verbs (katsoin and tyyntyi) are in the imperfect tense.

Why is the object merta in the partitive case here?

The verb katsoa (to look at, to watch) is a partitive verb. Whenever you watch or look at something, the object being looked at must take the partitive case. Furthermore, watching the sea is a continuous, ongoing action, which is a classic trigger for the partitive.

Is the verb tyyntyi related to the adjective for calm?

Yes! It comes from the adjective tyyni (calm). By adding the -tyä or -tua suffix, Finnish creates verbs that describe changing into a certain state. Therefore, tyyntyä literally means to become calm, and tyyntyi is its past tense form.