Learn Finnish: Opiskelijalla on tapana hypistellä kynää alitajuisesti. - The student has a habit of fiddling with a pen subconsciously.

The structure uses the adessive case for the person who has the habit (opiskelijalla), followed by on tapana (literally 'is as a custom'), and then the first infinitive of the action verb (hypistellä). Notice that the verb on always stays in the third-person singular in this construction.

What kind of verb is hypistellä?

It is a frequentative verb, meaning it describes a repeated, continuous, or casual action. The ending -ella or -ellä often adds this nuance. Here, it expresses continuous 'fiddling' or 'twiddling' rather than a single distinct movement.

Why is the object kynää in the partitive case?

The partitive is used because the action of fiddling (hypistellä) is an ongoing process without a clear endpoint. Because the action does not produce a final, completed result, its object must be in the partitive case.

How is the adverb alitajuisesti formed?

It is built from the prefix ali- (under or sub) and the noun taju (sense or consciousness). Adding the suffix -inen creates the adjective alitajuinen (subconscious), and replacing the -nen ending with -sesti turns it into the adverb 'subconsciously'.