Breakdown of Netko kuca na vrata.
na
on, at / to, onto (with motion) / about (thinking about)
netko
someone
kucati
to knock
vrata
door
Questions & Answers about Netko kuca na vrata.
Why does the verb kuca take the third-person singular form?
Because the subject is the indefinite pronoun netko (someone). Just like in English where we say 'someone is' instead of 'someone are', netko is always treated as grammatically singular and takes a third-person singular verb.
Could I also use netko to ask 'Is anyone knocking on the door?'
Yes! Unlike English, which often switches from 'someone' in statements to 'anyone' in questions, Croatian uses netko for both. If you turn this sentence into a question (Kuca li netko na vrata?), netko translates perfectly to 'anyone'.
Is vrata in the nominative case here?
No, it is in the accusative case. The preposition na takes the accusative when it shows the target of an action. Knocking is directed at the door, so we use the accusative. The accusative form of vrata just happens to look exactly the same as the nominative.
Since vrata is plural, why does the English translation say 'door' instead of 'doors'?
In Croatian, vrata is a 'plural-only' noun. It is grammatically a neuter plural word, but in the real world, it usually refers to a single door. You just have to treat it as a plural word in Croatian, even when talking about one door.