When should I use the subjuntive?

The subjunctive is used a lot in Spanish, and in many different instances. It is used to express doubt, wanting someone to do something or something to happen, giving an order, expressing a negative opinion, and hypothetical situations. For example:

No creo que tenga bastante dinero. (Doubt)

Quiero que vengas. (Wanting)

¡Qué te calles! (Order)

No comes en la sala de clase. (Negative order)

No creo que le guste viajar. (Negative opinion)

Ojalá que pudiera ir al concierto. (Hypothetical)

It's a good idea to know the grammar behind the subjunctive, but practising using phrases like these, and recognising when it is being used correctly, is the best way to fully understand its use.