Learn Croatian: Jesu li oni studenti?

Je li is used for singular subjects (he, she, it). Because the subject here is oni (they), we have to use the plural form of the verb to be, which is jesu.

What does the word li actually mean in this sentence?

The word li doesn't have a direct translation in English. It is a question word (a particle) used right after a verb to signal that the sentence is a yes/no question.

Can I drop the word oni and just ask Jesu li studenti?

Yes, absolutely! Croatian often leaves out pronouns like oni because the verb form (jesu) already tells us you mean they. Including oni here just adds a little emphasis or clarity.

Why does studenti end in an i?

Because the subject is plural (they), the noun describing them must also be plural. In Croatian, most masculine nouns like student become plural by adding an -i at the end.