[AngelicaDeAlquezar]

@dp444: Go through Rusty's explanations again and check his examples with "antes de que + subjunctive". That's the only correct way to express what you want to say with the conjugated verb. It cannot be used in the preterite and it cannot be used without the conjunction "que". Since your sentence is written in the past, then the corresponding subjunctive is also in the past: "llegaras".
The alternative is, as his last examples show, with a noun instead of the verb.