The main uses of the subjunctive include:
1. After the verb querer when there is a change of subject (but use the infinitive if there is no change of subject)
2. When one person tells (decir) or asks (pedir) another person to do something.
3. After expressions of emotion, such as esperar (to hope), sentir (to be sorry), temer (to fear), alegarse (to be glad), when there is a change of subject.
4. After dudar (to doubt) and other verbs expressing uncertainty.
5. After most impersonal expressions, such as es posible (it's possible), es importante (it's important), es necesario (it's necessary) if there is a subject for the subordinate verb.
6. In adjective clauses is the antecedent is indefinite
7. After certain conjunctions, such as para que (in order that), sin que (without), and antes que (before)
8. After time conjunctions, such as cuando (when), en cuanto (as soon as), hasta que (until), when futurity is implied.
9. In contrary-to-fact conditions, the past subjunctive must be used in the if-clause (and the main clause is in a conditional tense)
Other Perfect Tenses
Beside the present and past perfect tenses, there are also the preterite, future and conditional perfect tenses. All are conjugated with a form of haber and a past participle.
The preterite perfect is formed with the preterite of haber + past participle, and it has the same meaning as the past perfect. But this tense is normally only used after conjunctions of time, such as así que, luego que, tan pronto como (as soon as); cuando (when); después (de) que (after); and hasta que (until).
The future perfect is formed with the future of haber + past participle and is also used to express probability, referring to the present.
The conditional perfect is formed with the conditional of haber + past participle and is also used to express probability, referring to the past.
