When do I act upon certainty, and when do I act upon the predominant assumption?

Acting upon certainty is the default, and one acts upon the preponderant opinion when certainty is unattainable. Preponderant opinion is the apprehension of the more probable side, while certainty is a definitive apprehension based on conclusive evidence. In prayer, whoever doubts but has an opinion and a preference should act upon their preponderant opinion and perform the prostration of forgetfulness (sujud al-sahw) after the salām. As for one who has no preponderant opinion, they should act upon certainty, which is the lesser amount, and perform the prostration of forgetfulness before the salām. The Sunnah has indicated this detailed explanation.