Learn Finnish: Uusia kaapeleita asennettaessa liikennemerkki ohjaa kiertotielle. - When new cables are being installed, the traffic sign directs to the detour.

This is the passive temporal construction, meaning 'when something is done.' It comes from the verb asentaa (to install). To form it, take the passive stem asennetta- (from the passive present asennetaan) and add the inessive ending -essa. So, asennettaessa means 'when they are being installed'.

Why is Uusia kaapeleita in the partitive plural?

In a passive temporal construction, the object takes the same case it would in a normal passive sentence. Because installing the cables is an ongoing, incomplete process here, the object uusia kaapeleita must be in the partitive.

Why does the sentence use the allative case kiertotielle instead of the illative kiertotiehen?

In Finnish, words for roads, streets, and yards usually take the external location cases (the l-cases like -lla, -lta, -lle). The traffic sign directs you onto the road (tielle), not inside it.

What is the base form of the verb ohjaa?

The base form is ohjata (to direct or steer), which is a Type 4 verb. When conjugated in the third-person singular (se ohjaa), the consonant t disappears due to consonant gradation, and the final vowel is doubled.