When and why should I use the subjunctive in Spanish?

Firstly remember that the subjunctive is not a tense but a mood and is used for expressions of doubt and improbability. Here is an acronym to remember when to use the subjunctive (WEIRDO): Wishes, Emotions, Impersonal expressions, Recommendations, Doubt/Denial, Ojalá.There are three requirements for the subjunctive to be triggered: Firstly, the main clause and the subordinate clause will have different subjects (ie. the first and second part of a sentence). For instance: Quiero que pongas la mesa (I want you to lay the table). Secondly, a relative pronoun such as que, como, quien. 'Que' is the most common. Thirdly, two verbs, one for each subject. The first verb is always in the indicative and the second in subjunctive