Learn Finnish: Ei voi raapia silmiä jatkuvasti. - One cannot scratch one's eyes constantly.

You would use the adessive pronoun (minulla) + on tapana + the 1st infinitive of the main action verb (raapia). The modal verb voi is removed, and the sentence becomes: Minulla on tapana raapia silmiä jatkuvasti.

Why isn't there a word for "one" or "you" in Ei voi raapia?

This is the Finnish "zero person" construction, used for general statements like "one cannot" or "you shouldn't" (similar to the generic "you" in English). To form it, you simply leave out the subject pronoun and use the third-person singular verb form. In a negative sentence, this means using ei plus the bare stem of the modal verb (voi).

Why is silmiä in the partitive plural instead of the nominative or accusative plural?

Two rules overlap here to make the object partitive. First, the sentence is negative (ei voi), which always forces a partitive object. Second, the verb raapia (to scratch) is a continuous, irresultative action that inherently requires a partitive object even in positive sentences.

What is the root of the adverb jatkuvasti (constantly)?

It stems from the verb jatkua (to continue). The present active participle form is jatkuva (continuing / constant). By adding the common adverbial suffix -sti to the adjective stem, it turns into jatkuvasti (constantly).