Learn Croatian: Samo četvero ljudi može ući u vagon. - Only four people can enter the carriage.

Četvero is a collective numeral. In Croatian, collective numerals are used when talking about groups of children (djeca) or mixed-gender groups of people (ljudi). Since we don't know the specific genders of the people entering the carriage, or we assume it's a mixed group, četvero is the natural choice.

What case is ljudi in, and why?

Ljudi is in the genitive plural. This is a strict rule: collective numerals like dvoje, troje, and četvero always require the noun that follows them to be in the genitive case. Think of it a bit like saying "a group of four of people".

Why is the verb može singular when there are four people?

This is a very common trap! When a collective numeral like četvero is the subject of a sentence, the verb must be in the 3rd person singular. Grammatically, the language treats the group as a single unit, so we use može ("it can") rather than the plural mogu ("they can").

Why does u vagon use the accusative case here instead of the locative?