Learn Finnish: Oletko valmis kuorimaan sipulin? - Are you ready to peel the onion?

The adjective valmis (ready) dictates the form of the verb that follows it. It requires the third infinitive in the illative case, which ends in -maan or -mään. Think of it literally as being 'ready into peeling'. Saying 'valmis kuoria' is grammatically incorrect in Finnish.

Why is the object sipulin in the genitive-looking -n case instead of the partitive?

Peeling an onion is treated as a bounded, completed action here—you intend to finish peeling the entire onion. This requires a total object, which for a singular noun takes the -n ending. If you meant 'peel some onion' or if the action was ongoing, you would use the partitive sipulia.

Are there other adjectives that behave like valmis and require the -maan / -mään verb form?

Yes! Many adjectives expressing willingness, intention, or capability trigger this same rule. Common B2-level examples include halukas (willing), as in halukas auttamaan (willing to help), and kykenevä (capable), as in kykenevä tekemään (capable of doing).