grammar.subjunctive.overview
When the subjunctive fires: the map
The subjunctive appears in subordinate clauses after triggers of will, emotion, doubt, and unreality — typically with que and a change of subject: quiero que vengas.
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Explicación
The subjunctive isn't a tense but a mood, and it appears mostly in subordinate clauses after a 'trigger' in the main clause — typically with que and a change of subject: quiero que vengas, me alegra que estés. It marks information that is wanted, doubted, felt about, or otherwise not asserted as fact.
The main trigger families, each with its own rule: wishes and influence (quiero que, te pido que), emotion (me alegra que), doubt and denial (no creo que, dudo que), impersonal judgments (es importante que), purpose (para que), pending future time (cuando llegues), unknown or non-existent antecedents (busco a alguien que sepa…), and ojalá / quizás.
Two anchors keep it in check. With the same subject, Spanish usually uses the infinitive instead: quiero ir, not *quiero que yo vaya. And assertion of fact keeps the indicative: creo que viene, es verdad que llegó, sé que funciona.
Ejemplos
I want you to think it over.
Región: global
I think he's right.
Región: global
I want you to think about it, though I believe you already know the answer.