Under what circumstances is the present or imperfect subjunctive used?

Present subjunctive is used when subjunctive setup is in present or future tenses. For example, "Quiero que me hagas un favor", or "Voy a querer que me hagas un favor". Imperfect subjunctive is used when the subjunctive setup is in the past or conditional tenses. For example, "Le pedí que me hiciera un favor", or "Pediría que me hiciera un favor". Imperfect subjunctive is also used when speculating or speaking of hypothetical situations. For example, "Ojalá tuviera una casa al lado de la playa", or "Si no hubiera ido a la la universidad..."However, these circumstances could also lead to a present subjunctive if one were talking about a current or future situation. For example "Ojalá haga calor hoy".