486. Forgetting Insertions, or Bentching Altogether

Brachos 2:13

Let’s say that someone is bentching on Rosh Chodesh and forgets to recite yaaleh v’yavo. If he remembers before starting the fourth bracha, he says “Blessed is the One Who granted Rosh Chodesh to His people Israel as a remembrance.” The text does not have a concluding clause. He then starts the fourth bracha and finishes bentching. If he does not realize his error until after he has started the fourth bracha, he just completes that bracha and he need not repeat bentching. The same procedure is followed on chol hamoed. If one forgets al hanissim on Chanukah or Purim, he does not repeat bentching.

Brachos 2:14                    

Let’s say that someone ate and forgot to bentch. If he remembers before his food is digested, he should go back to where he ate and bentch. If he only remembers after his food has been digested, he does not go back and bentch. If one is unsure whether or not he bentched, he must go back and bentch so long as his food has not been digested.