Learn Dutch: Weet jij waarom iedereen, behalve de baas, een helm draagt? - Do you know why everyone, except the boss, is wearing a helmet?

behalve means "except" in this context. Here, behalve de baas acts as an extra piece of information inserted right after the subject (iedereen). The commas are used just like in English to show it is an aside that interrupts the main flow of the clause without changing the sentence structure.

Why is the verb draagt at the very end of the sentence?

The word waarom (why) introduces a subordinate clause (a dependent clause). In Dutch, a subordinate clause always pushes its working verb to the very end. The main sentence is just the short question Weet jij, and everything that follows is dependent on it.

The sentence mentions both "everyone" and "the boss". Why is the verb draagt singular rather than plural?

The grammatical subject of the subordinate clause is iedereen (everyone). Just like in English, iedereen takes a singular verb. The phrase behalve de baas (except the boss) is simply an exception to the subject, so it doesn't turn the subject into a plural group.

Why use jij instead of the shorter je?

Using the stressed pronoun jij places emphasis on the person you are talking to. It implies "Do you happen to know..." or "Are you the one who knows...", perhaps because the speaker expects the listener to have some inside information.