The basic pattern
This is what English calls the third conditional: a condition in the past that was not met, and the consequence that therefore never happened. The two languages line up almost word for word: had known is hubiera sabido, and would have come is habría venido, where one Spanish word does the work of would + have.
In the si clause haber is hubiera, hubieras, hubiera, hubiéramos, hubierais, hubieran (also with -se: hubiese). In the main clause it is habría, habrías, habría, habríamos, habríais, habrían. The participle never changes.
How the pattern sounds
Si hubiera sabido la verdad, habría venido.
If I had known the truth, I would have come.
Si hubieras estudiado, habrías aprobado.
If you had studied, you would have passed.
What type 3 is used for
Mostly you look back: you regret a decision of your own, reproach someone else, or imagine how things could have gone differently. The structure is always the same.
Si hubiera estudiado más, habría sacado mejor nota.
If I had studied more, I would have got a better grade.
Regret
Si me hubieras avisado, habría venido antes.
If you had let me know, I would have come earlier.
Reproach
Si no hubiera llovido, habríamos ido a la playa.
If it hadn't rained, we would have gone to the beach.
A different course of events
Word order and the main clause
As in English, the si clause can also come second. Then there is no comma.
¿Qué habrías hecho si hubieras estado en mi lugar?
What would you have done if you had been in my place?
No lo habría conseguido si no me hubieras ayudado.
I wouldn't have managed it if you hadn't helped me.
In the main clause, hubiera often replaces habría, especially in speech: Si lo hubiera sabido, te hubiera llamado (If I had known, I would have called you). The meaning is the same, and English cannot mirror the difference.
