The subjunctive must be used with unrealized or hypothetical events, situations, or entities. Pay close attention when you encounter any of the terms listed below.

cuando, en cuanto, mientras, hasta, antes de que, a menos que, con tal que, sin que, aunque, etc.

Le hablaré cuando vuelva.
I will speak to him when he returns.

Llámame en cuanto llegues.
Call me as soon as you get there.

Karina termina su trabajo antes de que venga a la clase.
Karina finishes her work before coming to the class.

No, no me hablan a menos que les hable yo a ellos.
No, they don’t speak to me unless I speak to them.

Mientras vivas en Madrid, no tendrás problemas de alojamiento.
While you live in Madrid, you won't have problems with accomodation.

Aunque esté amueblado, es barato.
Although it's furnished, it's cheap.

Aunque la mona se vista de seda, mona se queda.
Although the monkey is dressed in silk, she's still a monkey.

Note: The subjunctive is used only when speaking of hypothetical situations - not existing situations. When refering to past actions, or habitual actions, one uses the indicative.

Past Actions - indicative
¿Cuándo perdió su camara?
When did you lose your camera?

Habitual Actions - indicative
¿Cuándo piensa regresar?
When do you think you will be back?

Future Actions - subjunctive
Cuando sea grande quiero ser doctora.
When I grow up I want to be a doctor.