The Subjunctivisor!
Does tan pronto como require the subjunctive?
It depends. When the main clause is a command or in the (potential) future, tan pronto como requires the subjunctive:
Tan pronto como llegue el correo, dígame.
As soon as the mail arrives, tell me.Tan pronto como llegue el correo, la secretaria le diría.
As soon as mail arrives, the secretary will tell you.
But tan pronto como takes the indicative when the action in the subordinate clause is either habitual or in the past:
Tan pronto como llega el correo, la secretaria lo distribuye.
As soon as the mail arrives, the secretery distributes it.Tan pronto como llegó el correo, la secretaria lo distribuyó.
As soon as the mail arrived, the secretary distributed it.