Learn Finnish: Tuo on kirja.

Finnish doesn't use articles like "a", "an", or "the". The word kirja can mean "a book" or "the book" depending on the context. Here, Tuo on kirja simply translates to "That is a book."

I learned that se also means "that" or "it". Why use tuo here instead of se?

Tuo is used to point out something specific you can physically see or point to right now, like saying "that book over there." Se is usually translated as "it," and while it can sometimes mean "that," it is mostly used for things that are out of sight or were already mentioned in the conversation.

Does the verb on always go in the middle of sentences like this?

Yes, in a basic statement equating two things (like "A is B"), Finnish uses the exact same word order as English. "That is a book" maps directly word-for-word to Tuo on kirja.

Is kirja in its basic dictionary form here?

Yes! It is in the basic dictionary form (called the nominative). When you use the verb on to say what a single thing is, the noun following it usually stays in this unvaried basic form.