grammar.clauses.si-past-unreal
Lost pasts: si hubiera sabido, habría venido
Unreal past conditions: si + pluperfect subjunctive + conditional perfect: si hubiera sabido, no habría firmado. Spoken Spanish often doubles the hubiera.
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Explanation
For conditions about the past that did not happen, the frame is si + pluperfect subjunctive (hubiera/hubiese + participle) in the if-clause and the conditional perfect (habría + participle) in the main clause: si hubiera sabido, no habría firmado; si hubiéramos comprado en 2020, habríamos ganado el doble.
Very commonly in speech, the main clause also uses hubiera instead of habría: si hubiera sabido, no hubiera venido — fully accepted and extremely frequent across the Spanish-speaking world.
The two halves can mix time frames when the consequence is present: si hubiera estudiado medicina, ahora ganaría más ('if I had studied medicine, I'd now earn more') — past condition, present result. Alone with ojalá, the pluperfect subjunctive expresses regret: ojalá hubiera venido.
Examples
If you had told me, I'd have waited for you.
Region: global
If it hadn't been for you, I wouldn't have made it.
Region: global
If you'd told me earlier, I would have changed the flight.