Learn Croatian: Jesu li oni žedni?

In Croatian, you cannot start a sentence with a short-form verb like su. To ask a yes/no question using this structure, you have to put the verb first. Therefore, you must use the full (or emphatic) form of the verb, which is jesu.

What is the purpose of the word li?

li is a question particle. By putting li right after the full verb (Jesu), you signal that the sentence is a yes/no question. It does the same job as swapping the subject and verb in English (changing "They are" to "Are they?").

Why does the word for thirsty end in -i (žedni) here?

Adjectives must match the gender and amount (singular or plural) of the noun or pronoun they describe. The base dictionary word for thirsty is žedan (masculine singular). Because oni (they) is plural, the adjective drops the "a" and takes the plural ending -i, becoming žedni.

Could I leave out oni and just ask Jesu li žedni?

Yes, absolutely! Because the verb form (jesu) and the plural adjective (žedni) already make it perfectly clear that you are talking about "them", the pronoun oni is optional. Including it just adds a little extra focus on exactly who is thirsty.