rekisteröidessään

The Finnish word "rekisteröidessään" is a verbal noun in the adessive case, functioning adverbially. It translates to "while registering" or "upon registering." This form is derived from the verb "rekisteröidä," which means "to register."

The structure of "rekisteröidessään" breaks down into "rekisteröi-" (the verb stem), "-dessä-" (the verbal noun suffix indicating an ongoing or simultaneous action), and "-än" (the possessive suffix, third person singular, which functions here to indicate the subject performing the action). Thus, it literally means something akin to "in its/their registering."

It is commonly used in sentences to describe an action or event that occurs at the same time as or immediately after the act of registration. For example, one might say "Hän huomasi virheen rekisteröidessään tunnuksensa" which translates to "He noticed an error while registering his account." The adessive case on the verbal noun gives it an adverbial sense, indicating the time or circumstance under which the main verb's action takes place. This grammatical construction is a common way to express concurrent actions in Finnish.