How do I use the subjunctive?

The subjunctive, rather than a tense, is used as a 'mood'. Certain phrases and purposes take the subjunctive. An acronym to remember what purposes take the subjunctive is "WEIRDO" which stands for "Wishes, Emotions, Impersonal expressions, Recommendations, Doubt/Denial, and Ojalá (I hope)".
To form the present tense subjunctive you take the infinitive - e.g. tener. Take the first person conjugation - tengo and remove the o to add the endings "a, as, a, amos, áis, an" to make 'tenga' (first person).For ar verbs you add the endings "e, es, e, emos, éis, en."
An example of where the subjunctive is used is "espero que tengas un buen día." which means I hope you have a good day.A clear indicator of the subjunctive is 'que' e.g: recomiendo que, es importante que, quiero que . . .