Learn Finnish: Uusi kampaus pehmentää kasvojasi. - The new hairstyle softens your face.

It comes from the adjective pehmeä (soft). By dropping the final vowel -ä and adding the transitive suffix -ntää, we get pehmentää, which means "to make (something) soft" or "to soften." This directly applies the pattern of turning an adjective into an active, state-changing verb.

Why is the object kasvojasi in the partitive case here instead of the accusative?

Verbs that describe a visual effect, an ongoing process, or an abstract influence usually take a partitive object. Because the hairstyle is continuously softening the face's appearance, rather than completing a literal physical transformation, kasvojasi must be in the partitive.

The English translation says "face" (singular), but kasvojasi is grammatically plural. Why?

The Finnish base word for face, kasvot, is a plural-only word (just like "scissors" or "glasses" in English). Because of this, its partitive form kasvoja is built in the plural. The -si at the end is simply the possessive suffix meaning "your".

Is the noun kampaus (hairstyle) derived from another word?

Yes, it is derived from the verb kammata (to comb). Many Finnish verbs ending in -ata / -ätä use the suffix -us / -ys to create a noun that represents the result of the action. Just as maalata (to paint) becomes maalaus (a painting), kammata becomes kampaus (a hairstyle).