How do I use the subjunctive in Spanish?

The subjunctive mood in Spanish is used when the speaker wishes to express doubt (e.g. Dudo que..), wishes (e.g. Espero que..), to make an impersonal statement (when they are not certain, e.g. 'Es probable que..'), and in a phrase where the action directly affects you (e.g. Me molesta que..). The subjunctive is used with some fixed expressions, such as 'Aunque..', 'A menos que' and 'Antes de que..', and also when a sentence or clause starts with 'Que' (e.g. Que tengas un buen día). It is also used in positive and negative commands. It takes the tú present form when using tú/vosotros for a negative demand (e.g. '!No olvides tu maleta!' / 'No comais nada') and the present usted form when using positive demands for usted/ustedes (e.g. 'Hable más despacio' / 'Vengan aquí').