rekisteröityivät

rekisteröityivät is the third-person plural past tense form of the Finnish verb "rekisteröityä," which means "to register." In Finnish grammar, this verb ending indicates that a group of people or entities performed the action of registering at some point in the past. The subject of the sentence would be plural, referring to "they" who registered. For example, a sentence might state "Oppilaat rekisteröityivät kurssille," meaning "The students registered for the course." The verb is used in contexts where an action of formal enrollment, sign-up, or official recording has taken place by multiple subjects. The base verb "rekisteröityä" itself is a passive or reflexive form derived from "rekisteröidä" (to register). Therefore, "rekisteröityivät" specifically conveys that "they" underwent the process of registration themselves or were registered by someone else, but the emphasis is on the plural subjects' involvement in the act of being registered. This grammatical form is crucial for understanding the timing and the number of participants in past events of registration within Finnish sentences.