Learn Hardcore Finnish: Kuitti on tärkeä.

Finnish has no articles. Words like “a” and “the” are simply omitted. Context tells you whether you mean any receipt (indefinite) or a specific receipt (definite). If you really need to emphasize definiteness, you can add a demonstrative: Tämä kuitti on tärkeä (“This receipt is important”).

What case is kuitti in, and why is it in that case?

Kuitti is in the nominative singular. In Finnish, the subject of a sentence (the “thing doing or being described”) appears in the nominative. Here, kuitti is the subject of the verb on (“is”), so it stays in the nominative form.

Why doesn’t the adjective tärkeä have any extra ending?

Adjectives agree in case and number with the noun they modify. Since tärkeä is a predicate adjective describing kuitti (nominative singular), it also takes the nominative singular form tärkeä. If you had multiple receipts, you’d say tärkeitä (nominative plural).

What is on, and how is it formed?

On is the third‐person singular present tense of the verb olla (“to be”).
– Minä olen (“I am”)
– Sinä olet (“you are”)
– Hän/se on (“he/she/it is”)
So in Kuitti on tärkeä, on simply means “is.”