Learn Finnish: Sitä mukaa kun olimme kairanneet kallioperää, melu oli pahentunut. - As we had drilled the bedrock, the noise had worsened.

While kun just means "when" or "as" in a general time sense, sitä mukaa kun emphasizes a proportional progression. It indicates that as the drilling progressed step-by-step, the noise grew worse at the exact same rate. It conveys the idea of "in proportion as" or "the more... the more...".

Both clauses use the past perfect tense (olimme kairanneet, oli pahentunut). Why use this instead of the simple past?

Using the past perfect (the "had done" tense) for both verbs emphasizes that this proportional, simultaneous change was an ongoing background process that happened before some other past event. In a larger story, this sentence sets the scene for what happened next. Simple past (kairasimme, paheni) would just state the actions as standalone past events.

Why is the object kallioperää in the partitive case rather than the total object (genitive/accusative) case?

The partitive case is used because the action of drilling (kairata) is an ongoing, incomplete process in this context. They were drilling into the bedrock, rather than drilling the entire bedrock to a finished state.

How is the verb pahentunut related to the word for "bad" (paha)?