if (someone) had known
Si hubiese sabido de tu enfermedad, podría haberte visitado en el hospital.
If I had known about your illness, I could have visited you in the hospital.
Estoy seguro de que Tom me lo hubiera dicho si hubiese sabido.
I'm sure Tom would have told me if he'd known.
Si hubiese sabido lo que estaba a punto de pasar, no habría hecho eso.
If I had known what was about to happen, I wouldn't have done that.
Si hubiese sabido que estabas aquí, habría venido inmediatamente.
If I had known you were here, I would have come immediately.