The subjunctive must be used with unrealized or hypothetical events, situations, or entities.

cuando, en cuanto, mientras, hasta, etc.

Future Actions - Unrealized Events (view lesson here)

Past Actions ---> Indicative
Habitual Actions ---> Indicative
Future Actions ---> Subjunctive

antes de que, a menos que, con tal que, sin que, aunque,, relative clauses (only hypothetical, not existing situations).

Todo estará perfecto cuando Ud. vuelva a casa.
Everything will be fine when you get home.

Quiero leer esta revista mientras mires tus telenovelas.
I want to read this magazine while you watch your soaps.

Aunque la mona se vista de seda, mona queda.
Even if the monkey gets dressed up in silk, she's still a monkey.
(You can't make a silk purse out of a sow's ear.)

Examples:

no es seguro que - it's doubtful that

No es seguro que venga.
It's doubtful that he'll come.

es posible que - it's possible that

Es posible que vayamos a la playa mañana.
It's possible that we'll go to the beach tomorrow.

es probable que - it's likely that

Es probable que llueva mañana.
It's likely to rain tomorrow.

hasta que - until

Tienes que quedar hasta que lo hayas terminado.
You have stay until you've finished it.

When these verbs refer to the future or the unknown they
are followed by the subjunctive. When they express something
habitual you don't use the subjunctive.

Me matará cuando se entere.
He'll kill me when he finds out.

BUT

Siempre nos ayuda cuando es necesario.
She always helps us when necessary.