Past Perfect (e.g.: había hablado)

Questions about the Past Perfect

 Q: Why is it Si lo había visto ... and NOT Si lo habría visto?

After the word si, you can use the present, the imperfect or the future. However, si CANNOT be directly followed by the conditional or the past conditional. It is true that English uses the structure "would have" , e.g.: "If I would have known", but this can't be a conditional in Spanish. Rather, you should use the past perfect subjunctive, e.g.: Si hubiera sabido.