Learn Croatian: Tko je prvi u redu? - Who is first in the queue?

Ordinal numbers like prvi act like adjectives, meaning they change their endings to match gender. When asking a general question with tko (who) about an unknown person, Croatian automatically defaults to the masculine singular. If you were looking at a line made entirely of women, you would ask Tko je prva?

Why does the word for queue end in -u (u redu)?

The dictionary form of the word is red (a masculine noun). Because we are describing a static location (being in the queue), the preposition u requires the locative case. For most masculine nouns, forming the locative means adding an -u to the end.

Does red always translate to "queue"?

Not always. Red basically means "order", "row", or "line". While it means "queue" when people are waiting for something, you will also often hear the exact phrase u redu used to mean "alright" or "okay" (literally: in order).

Why couldn't we start the sentence with je?

The word je is a clitic, which is a short, "weak" word without its own accent. A strict rule in Croatian is that clitics can never be the very first word in a sentence. They prefer the second position, which is why it sits right after Tko.