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1. The Gemara explains how the blessing of Boneh Yerushalayim ends.
2. The Gemara teaches what one should do if he forgets to insert Retzeh or Ya'aleh v'Yavo into Birkas ha'Mazon.
3. The Gemara discusses the Halachah in the cases of a person who forgets Ya'aleh v'Yavo on Rosh Chodesh in Shemoneh Esreh and in Birkas ha'Mazon.
4. The Gemara explains the reason for these laws (#3).
5. The Mishnah and Gemara discuss the different types of Zimun.
A BIT MORE
1. While one opinion says that the blessing concludes with "Moshi'a Yisrael,' Rav Nachman says that the blessing should conclude with the words, "Boneh Yerushalayim."
2. If one already started to say the blessing of ha'Tov veha'Metiv, he must go back and start Birkas ha'Mazon from the beginning. If he did not yet start ha'Tov veha'Metiv, then he should recite at that point the appropriate blessing which the Gemara teaches for that particular day.
3. If a person forgets to say Ya'aleh v'Yavo in Shemoneh Esreh of Shacharis or Minchah on Rosh Chodesh, he must repeat Shemoneh Esreh. However, if he forgot to say it in Birkas ha'Mazon, he does not repeat Birkas ha'Mazon.
4. Since one is obligated to say Shemoneh Esreh on Rosh Chodesh, when he forgets it he is omitting something that he is obligated to say. In contrast, one is not obligated to eat bread on Rosh Chodesh, and therefore if he omits mention of Rosh Chodesh in Birkas ha'Mazon he does not repeat Birkas ha'Mazon.
5. For example, a Zimun of three does not mention "to Elokeinu" in the blessing of Zimun, while a Zimun of ten does. There is a dispute in the Mishnah about whether the Zimun for any amount over ten is the same as the Zimun for ten, or whether more praises added when there are 100 people, 1000 people, and so on.
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