When should I use the subjunctive?

The subjunctive is used to express desires, wishes, uncertainties, possibilities and doubts. For example, in Spanish when you wish to say 'I don't think it is sunny', you would use the subjunctive (No creo que esté caliente).Meanwhile the indicative is used to express facts and statements which are believed to be true. For example, 'it is hot outside,' you would use the indicative (hace calor afuera).