Learn Finnish: Uutta viemäriä rakennettaessa putki meni rikki, mikä aiheutti vaaran. - When a new sewer was being built, a pipe broke, which caused danger.

It is the passive temporal construction, which translates to "when something was being built." You form it by taking the passive past participle, which is rakennettu for the verb rakentaa. You drop the final vowel and add the passive inessive ending -aessa or -äessä. It describes an action happening at the same time as the main clause, performed by an unspecified group or person.

Why is Uutta viemäriä in the partitive case instead of the nominative?

With the passive temporal construction, the object keeps the same case it would have in a regular passive sentence. Because the building of the sewer was an ongoing, incomplete process at the moment the pipe broke, it requires the partitive case.

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

In Finnish, joka refers back to the specific noun right before it. However, mikä is used when "which" refers to the entire preceding clause or situation. Here, it was the whole event of the pipe breaking that caused the danger, not just the word rikki, so mikä is the correct choice.

What does meni rikki mean, and why not use a single verb for "broke"?