When To Not Use Subject Pronouns In Spanish
In Spanish the verb forms often make subject pronouns unnecessary, and when that's the case the pronouns shouldn't be used unless there's a reason to. Natives rarely start their sentences with subject pronouns.
Sampling of sentences where pronouns are unnecessary because the verb forms make clear who is performing the action of the verb:
Tengo sueño. I feel sleepy. (The verb "tengo" can refer only to the person speaking so using "yo" is unnecessary).
¿Dónde has estado? Where have you been? (The verb "has" necessarily refers to "tu").
Hemos perdido el tren. We missed the train. (Hemos = Nosotros hemos).
Miguel no está en la oficina. ¿Está de vacaciones? Miguel isn't at the office. Is he on holidays? (The context shows that ''¿Está de vacaciones?'" refers to Miguel, so saying "¿Está Miguel de vacaciones?" isn't necessary).