Use of subjunctive in "show battery state when..." "battery is present"

Question

I noticed that on my Ubuntu machine, there's a setting labeled

Mostrar el estado de la batería en la barra de menús

with dropdown options:

• Cuando esté presente la batería
• Cuando la batería esté cargandose o en uso
• Nunca

My question is, why do these appear to be in the subjunctive? I'm studying Spanish at an intermediate student level in school, and from what I've been taught, I'd write these using indicative, similar to "Cuando está presente la batería".

Can someone shed some light on this? Is there a rule (in certain locales, perhaps) that selects for subjunctive for such phrases?

Answer 1

Subjunctive is used in this case because it is trying to express uncertainty. It is uncertain that the battery will be present in the future, so the subjunctive mood should be used.

It is often used after the word "cuando" when talking about a possible future event. For example, "Voy a ir de vacaciones, cuando tenga mas dinero." Indicative should be used after the word "cuando" when talking about an event that has already occurred or is occurring in the present, for example, "No se cuando fue la última vez que fui de vacaciones."

This explanation has a lot more examples: http://spanish.about.com/od/verbmoods/a/cuando-subjunctive.htm