Unrealized or hypothetical situations
When the main verb is in the future tense and the relative clause is introduced by one of the following conjunctions, always use a subjunctive verb in the relative clause.EN CUANTO
Te haré llegar los informes en cuanto los tenga preparados.
I'll send you the reports as soon as I get them ready.
MIENTRAS
Tomaré una ducha mientras prepares la cena.
I'll take a shower while you make dinner.