Learn Croatian: Oni su važni ljudi. - They are important people.

The word važni (important) is an adjective describing ljudi (people). Since ljudi is a masculine plural noun, the adjective must match its gender and number. For masculine plural nouns in the nominative case (when they are the subject of the sentence), adjectives take the -i ending. The singular form for a masculine subject would be važan.

I don't recognize the noun ljudi. What is its singular form?

The word ljudi means people and is a highly irregular plural noun. Its singular form is čovjek (man or human). Even though they look completely different, they work together exactly like person and people do in English. Grammatically, ljudi acts as a masculine plural noun, which is why the adjective before it must be važni.

Why do we use oni for they instead of one?

In Croatian, the word for they changes depending on who or what you are talking about. You use oni for masculine or mixed-gender groups. Since the noun ljudi (people) is grammatically masculine, we use oni. If the sentence was about women (žene), you would use the feminine plural pronoun one.

Can I change the word order and say Oni važni ljudi su?