If one forgot to inset "Al Hanisim" in the Amidah, he does not have to repeat the Amidah. However, if he remembers before the completion of the Amidah, he should insert it after "Yeh'Yu L'Rason," before "Oseh Shalom." This applies even if he remembered after saying "Baruch Atah Hashem," before concluding "Ha'tov Shimcha." He should not quickly conclude "Lamdeni Hukeha," but should finish the Beracha and insert "Al Hanisim" before Oseh Shalom.
There is a disagreement as to how to formulate the "Al Hanisim" at that point. Everyone agrees that if he forgot "Al Hanisim" in the Birkat Hamazon, he does not repeat Birkat Hamazon and should insert it as a "HaRahaman" prayer in the "HaRahaman section, as brought by the Rema in Siman 682. The Mishna Berura there teaches that the same is true in the Amidah-it should be inserted in the form of a supplication. However, the Ben Ish Hai in Hilchot Purim rules that it should be recited in its original form-"Al Hanisim…"
If one began a meal at the end of the last day of Hanukah and concluded it after nightfall, according to Maran, he should say "Al Hanisim." It is no different than Birkat Hamazon on Mosa'eh Shabbat, Mosa'eh Yom Tob or Mosa'eh Rosh Hodesh, in which the Birkat Hamazon is defined by the beginning of the meal. Although the Ben Ish Hai understood that this is not the case on Hanukah and Purim, The Minhag of Yerushalayim, Hacham Atiya, Hacham Ovadia and Hacham Bension all agree that one should say "Al Hanisim" after nightfall, preferably if he ate a Kezayit before sunset.
In Birkat Hamazon, the word "Migdol" supplants "Magdil" on a day on which there is Musaf. Although Hanukah and Purim do not have Musaf, the Ben Ish Hai rules that on Purim "Migdol" is said, because there is a special "He'arah" (spiritual illumination) on that day. Nevertheless, that would only apply to Purim, whereas on Hanukah the "He'arah" is only during the candle lighting, and therefore "Magdil" is said.
The full Hallel is recited all eight days. The Pele Yoes and others emphasize that since the Hallel was one of the primary institutions of Hanukah it should be recited slowly with melody and intent. Even on Pesah, the full Hallel is not recited all eight days. This indicates the magnitude of the Misva of Hallel on Hanukah. The Rambam indicates that women are not obligated to say the Hallel, and although they can recite it, they may not say a Beracha.
SUMMARY
If one remembered to say "Al Hanisim" after the proper place: In Birkat Hamazon, he should insert it as a "HaRahaman" in the "HaRahaman" section, and in the Amidah, he should insert it before "Oseh Shalom" in its original form.
If one started a meal at the end of Hanukah, he should recite "Al Hanisim," in the Birkat Hamazon, even if he concluded after nightfall.
In Birkat Hamazon, "Magdil" is said, not "Migdol."
Women who chose to say Hallel do not recite the Beracha.
