Along with en attendant que(“until”) and jusqu'à ce que(“until”), avant que is one of the three conjunctions of time that take the subjunctive in prescriptive French; all three refer to expected events in the future. Colloquially, après que may also take the subjunctive, but this is discouraged by language authorities.
In written French, the verb that follows avant que takes the expletive particle ne, which does not negate the verb and serves no grammatical function.
Il mourut avant que l'ambulance n'arrive. ― He died before the ambulance arrived.
