If an action has not yet occurred, a native speaker of Spanish will use the subjunctive after certain adverbial clauses such as: tan pronto como (‘as soon as’), cuando (‘when’), después de que (‘after’), en cuanto (‘as soon as’), hasta que (‘until’), and mientras (‘while,’ ‘as long as’).
 

If the action has already taken place and there is therefore no doubt about its outcome, the subjunctive is not used: Cuando estuve en Acapulco, fui a la playa (I went to Acapulco in the past).