Halacha Orach Chaim 188

If someone forgot to recite Retzei on Shabbos, or Yaaleh Veyavo on Yom Tov, the Mechaber in 188, 6 bring the proper Bracha that should be recited, on condition that the fourth Bracha of Hatov Vhameitiv was not started.

If someone is not sure if Retzei or Yaaleh Veyavo was recited, he must assume that it was omitted.

There is a discussion among the commentaries when we consider the fourth Bracha started. The Mishna Berura writes that if he just said Baruch Atah Hashem, he should conclude Lamdeini Chuckecha, and recite the appropriate Bracha in place of Retzei or Yalaleh Veyavo before continuing with the fourth Bracha of Benching.

If he already said the words Melech Haolom, and at that point remembered Retzei, the Halacha according to the Mishna Berura is that he should complete the Nusach of the Bracha that comes in place of Retzei. In the Biur Halacha, the Chofetz Chaim question how one can complete this Bracha in place of Hatov Vhameitiv, as he already removed himself from the third Bracha of Benching. Although the Chofetz Chaim does not resolve this issue, in the Mishna Berura he seems to accept the option of concluding with the Bracha for Shabbos or Yom Tov. Reb Shlomo Zalman Aurbach comments that this is the accepted custom.

Once the words Hatov Vhameitiv are recited, Birchas Hamazon must be repeated from the beginning.