Learn Finnish: Kuka penkoo minun reppuani? - Who is rummaging through my backpack?

You would use the 3rd-person singular form sattuu (happens to) and put the main verb in the -maan/-mään form, matching the pattern in your lesson's example. The sentence becomes: Kuka sattuu penkomaan minun reppuani?

Why is the object reppuani in the partitive case here?

The verb penkoa (to rummage, to root through) describes an ongoing, continuous process rather than an action with a clear endpoint or final result. This open-ended nature triggers the partitive case for its object, giving us reppua, before adding the possessive suffix -ni.

Is the pronoun minun strictly necessary since the word reppuani already has the -ni possessive suffix?

No, in standard written Finnish, the pronoun minun is optional because the suffix -ni already indicates ownership. Including it just adds emphasis, as in Who is rummaging through my backpack? However, in colloquial spoken Finnish, you will usually encounter the exact opposite: the pronoun is kept, often shortened to mun, and the suffix is dropped entirely, resulting in mun reppua.

What is the infinitive of penkoo, and how does its consonant gradation work?