Learn Finnish: Viemäriä kaivettaessa tapahtuu usein virheitä, mikä aiheuttaa ongelmia. - When a sewer is being dug, mistakes often happen, which causes problems.

This is the passive temporal construction, meaning "when [something] is being dug". To form it, take the present passive of kaivaa (to dig), which is kaivetaan. Drop the final -an to get the stem kaivetta-, and add the inessive ending -essa.

Why is viemäriä in the partitive case?

In a temporal construction, the word for the thing being acted upon (viemäriä) follows normal object rules. Because digging is described as an ongoing, continuous action here, the object takes the partitive case.

If virheitä (mistakes) is plural, why is the verb tapahtuu in the singular?

This acts like an existential sentence. The subject virheitä is in the partitive plural because it represents an indefinite amount ("some mistakes"). When a partitive subject follows the verb like this, the verb must stay in the third-person singular (tapahtuu).

Why does the sentence use mikä instead of joka for "which"?

You use mikä when the relative pronoun refers to an entire preceding clause or situation (the whole idea that mistakes happen during digging). If "which" referred to a specific noun right before it, you would use joka.