If a person ate an amount of bread requiring him to recite Birkat Ha'mazon, and now he is uncertain whether or not he had already fulfilled this obligation, must he recite Birkat Ha'mazon, or may he rely on the possibility that he had already recited it?
A basic principle in Halacha distinguishes with respect to Sefekot (cases of uncertainty) between laws that apply on the level of Torah obligation, and those that are required by force of Rabbinic enactment. When it comes to Torah obligations, we must act stringently in situations of Halachic doubt ("Safek De'orayta Le'humra"). With regard to laws enacted by the Sages, however, we may assume the more lenient possibility ("Safek De'rabbanan Le'kula").
Thus, when a person is uncertain whether or not he recited Birkat Ha'mazon, he must determine whether in his situation the obligation of Birkat Ha'mazon applies on the level of Torah law, or whether the meal he ate requires Birkat Ha'mazon only on the level of Rabbinic enactment. The Torah obligation of Birkat Ha'mazon applies when a person ate bread to the point of "Sevi'a," satiation, as clearly indicated in the verse where the Torah introduces this Misva: "You will eat and be satiated, and you will then bless Hashem your God" (Devarim 8:10). The Halachic authorities clarify that the Torah obligation obtains even if one became satiated only as a result of the combination of bread and other foods. So long as a person partook of a Ke'zayit of bread, if he feels satiated he is required to recite Birkat Ha'mazon on the level of Torah obligation. If, however, a person does not feel satiated, then assuming he partook of a Ke'zayit of bread he must still recite Birkat Ha'mazon, though on the level of Rabbinic enactment. The Torah obligation does not take effect in the absence of satiation, but the Sages nevertheless required that one recite Birkat Ha'mazon if he ate at least a Ke'zayit of bread.
Hence, the Halacha in situations of uncertainty depends on the particular circumstances. If the individual ate a Ke'zayit of bread and feels satiated – even if the satiation is the result of other foods eaten with the bread – and then if he cannot remember whether he recited Birkat Ha'mazon, he must assume that he had not recited it, and must therefore recite it at this point. He recites only the first three Berachot, which constitute the Torah obligation of Birkat Ha'mazon. If, however, he does not feel satiated, then in situations of doubt he may be lenient, and does not have to recite Birkat Ha'mazon. This is the ruling of the Kaf Ha'haim (Halachic work by Rabbi Yaakov Sofer, 1870-1939), in Siman 184 (15), and of Rabbi Moshe Halevi (Israel, 1961-2001), in his work Birkat Hashem (vol. 2, p. 283; listen to audio recording for precise citation).
It must be noted, however, that this Halacha applies only to men, given the question surrounding the nature of a woman's obligation in Birkat Ha'mazon generally. The Shulhan Aruch expresses uncertainty as to whether women are required to recite Birkat Ha'mazon on the level of Torah obligation, or by force of Rabbinic enactment. Hence, Rabbi Moshe Halevi ruled that when a woman cannot remember whether or not she recited Birkat Ha'mazon, in all circumstances she does not recite it. Since even if she had eaten to the point of satiation her obligation might not constitute a Torah obligation, she is allowed to assume that she had recited Birkat Ha'mazon, and thus is not required to recite it at this point.
Summary: If a woman cannot remember whether or not she recited Birkat Ha'mazon, she may assume that she had already recited it. If a man cannot remember whether or not he recited Birkat Ha'mazon, then if he had eaten to the point of satiation, he must now recite the first three Berachot of Birkat Ha'mazon; otherwise, he may assume that he had already recited it.
