Under what circumstances is the present or imperfect subjunctive used?

Present subjunctive is used when the subjunctive setup is in present or future tenses. For example "Quiero que me hagas un favor" or "Va a ser necesario que me hagas un favor". Imperfect subjunctive is used when the subjunctive setup is in past or conditional tenses. For example, "Le pedí que me hiciera un favor" or "Sería necesario que hablaras con él". Imperfect subjunctive can also be used when speculating or speaking of hypothetical situations. For example "Si tuviera una bicicleta llegaría a la oficina a tiempo".However when speaking with current or future context the present subjunctive is used. For example "Ojalá haga calor más tarde".