Learn Finnish: Hänellä on tapana naputtaa pöytää sormella. - He has a habit of tapping the table with a finger.

The structure literally translates to 'At him is as a habit'. Using the essive case ending (-na, meaning 'as a state or role') with the 'have' structure (hänellä on) is the standard Finnish way to express that someone customarily does something.

Why is pöytää in the partitive case rather than the accusative/genitive?

The verb naputtaa (to tap) expresses an ongoing, continuous action rather than an action with a clear, completed endpoint. Verbs describing this kind of irresultative activity take a partitive object.

English uses 'with a finger', so why don't we use the postposition kanssa (with) instead of the adessive case (sormella)?

The adessive case (-lla/-llä) is used to indicate the instrument or tool used to perform an action. While kanssa means 'with' in the sense of company or accompaniment (like 'with a friend'), the adessive is needed when you mean 'by means of' or 'using', such as tapping with a finger.

How does Hänellä on tapana naputtaa differ in nuance from simply saying Hän naputtaa yleensä?

While both describe something done regularly, Hänellä on tapana emphasizes the personal, ingrained nature of the habit. It is the perfect construction for describing someone's quirks, personal rules, or routines, whereas yleensä simply states that an action 'usually' happens.