Learn Hardcore Finnish: Kieli on uusi.

Finnish does not have articles at all—no equivalent of a or the. To say “the language,” you just say kieli. If you need to be more specific, you can use demonstratives such as tämä kieli (“this language”) or se kieli (“that language”).

Why is the verb on used here, and what does it mean?

On is the third-person singular form of the verb olla (“to be”). It is used for he/she/it forms in the present tense, as well as with nouns. So kieli on uusi literally means “language is new.”

Why is uusi in this basic form, instead of something like uutta?

In Finnish, adjectives often agree with the noun in case and number. Because kieli is in the nominative case and we’re simply describing its condition (it is new), uusi stays in the nominative form. The partitive form uutta might appear in other contexts (e.g., if you were saying “I’m learning something new,” opin jotain uutta).

Is the word order flexible? Could one say Uusi on kieli?

Although Finnish word order is more flexible than English, Uusi on kieli would sound unusual or emphasize the word uusi in a poetic or very stylized way. Standard usage for a statement like “The language is new” would be Kieli on uusi.