Learn Croatian: Kupuju moderno računalo. - They are buying a modern computer.

In Croatian, adjectives must match the gender, number, and case of the noun they describe. The noun računalo (computer) is neuter. Because it is the direct object being bought, it is in the accusative case. For neuter singular nouns, the accusative looks exactly like the dictionary form, so the adjective uses the standard neuter ending -o.

Did the word računalo change its ending because it is the direct object?

No, it stayed exactly the same! Neuter nouns like računalo share the same form for both the subject of a sentence (nominative case) and the direct object (accusative case).

Does the fact that a computer is inanimate (not alive) affect the adjective moderno?

Not for this word. While the current lesson talks about inanimate objects, the alive/not alive distinction only changes the accusative endings for masculine nouns. For neuter nouns like računalo, the accusative form is always the same as the dictionary form, regardless of whether the object is alive or not.

Where is the word for "they" in Kupuju moderno računalo?

The pronoun oni (they) is left out because the verb ending already tells us who is doing the action. The -ju ending on kupuju means "they are buying," so adding oni isn't necessary unless you want to emphasize it.