Learn Finnish: Nämä asiakirjat kuuluvat näihin kansioihin. - These documents belong in these folders.

The word tämä (this) has to match the case of the noun it modifies. Since the folders are in the illative plural case (kansioihin), "these" must also be in the illative plural, which is näihin. At the beginning of the sentence, "these documents" is the subject, so it uses the basic nominative plural nämä.

Why do we use the "into" case (näihin kansioihin) with the verb kuulua?

The verb kuulua (to belong) requires the illative ("into") case to express where something belongs. While English says "belongs in" or "belongs to", Finnish conceptually treats belonging as placing an object into its proper location.

How do you build the illative plural word kansioihin?

You start with the base word kansio (folder). To make it plural, add the plural -i- marker, which gives the stem kansioi-. Finally, add the illative ending -hin to get kansioihin (into the folders).

Why does the verb end in -vat?

Nämä asiakirjat (these documents) is a plural subject. Just like you would say "he kuuluvat" (they belong) for people, you must use the third-person plural ending -vat for plural objects so the verb agrees with the subject.