3 Main Uses A - In many sentences where 2 clauses are joined by "que" Espero que comas bien. I hope you eat well. The subjects of the 2 sentences must be different so you can use a subjunctive in the 2nd clause. B - After certain conjugations(when, before, although, etc.) where the verb is in doubt or has not happened yet. Cuando vengas a mi casa, vamos a cenar When you come to my house, we'll eat
C - In certain conditional sentences where future event is unlikely or a past event that has never happened. Si pudiese, viviría en Santiago. If I could, I would live in Santiago. Si hubiera sabido, habría ido contigo. If I had known, I would have gone with you.
In a sentence with 2 clauses joined by "que" There must be different subjects doing the verbs. What happens in the first clause makes the verb in the second clause subjunctive. A - Influencing other people Insisto en que escribas una carta. I insist that you write a letter. B - Expressing Doubt Dudo que llegue a tiempo. I doubt that he'll arrive on time. C - Expressing Emotion Me alegro que puedas venir. I'm glad that you can come. D - Expressing Opinion No creo que seas la persona adecuada para el trabajo. I don't believe you are the right person for the job. E - Expressing Possibility Es possible que no coma nada carne. I'ts possible that she doesn'nt eat any meat.
F - Indefinite Antecedent Buscan un hombre que hable inglés. They are looking for a man that speaks English.
After certain conjugations(when, before, although, etc.) where the verb is in doubt or has not happened yet. Llámame en cuanto llegue. Call me as soon as he arrives. Mientras me pague, no me importa. As long as he pays me, I don't mind.