The recitation of Birkat Ha’mazon must include certain themes in order to qualify as a valid Birkat Ha’mazon. Specifically, one must make mention of God’s provision of food, the Land of Israel, Jerusalem and the Bet Ha’mikdash. The first three Berachot of Birkat Ha’mazon are recited for the purpose of mentioning all these themes. The first Beracha praises God for providing all creatures with food, the second expresses gratitude to God for giving us the Land of Israel, and the third prays for the rebuilding of Jerusalem and the Bet Ha’mikdash.
There is a debate among the Halachic authorities as to whether or not these three distinct blessings are required by force of Torah obligation, or only on the level of Rabbinic enactment. Maran (author of the Shulhan Aruch), both in Bet Yosef (Orah Haim 191) and in Kesef Mishne (Hilchot Berachot, chapter 2), maintains that as far as the Torah obligation is concerned, one may incorporate these themes in a single Beracha. In his view, it was Hazal who enacted that each theme should occupy a distinct Beracha; on the level of Torah obligation, one fulfills the Misva by reciting even one Beracha that mentions all these concepts. Tosefot (Talmud commentaries by Medieval French and German scholars), by contrast, held that even Torah law requires the recitation of three distinct Berachot.
The practical difference between these two views arises in a case of a person who ate an amount of bread requiring Birkat Ha’mazon, but mistakenly recited Al Ha’mihya (the Beracha recited after eating other baked foods). Al Ha’mihya consists of only one Beracha, but includes within it all the themes of the first three Berachot of Birkat Ha’mazon. According to Maran, then, one would not have to recite Birkat Ha’mazon in this case. Since he has fulfilled his Torah obligation by reciting Al Ha’mihya, he would not then be required to recite Birkat Ha’mazon. According to Tosefot, however, this individual did not even fulfill the Torah obligation, and would therefore be required to recite Birkat Ha’mazon.
We, of course, follow the view of Maran, and therefore one who mistakenly recites Al Ha’mihya instead of Birkat Ha’mazon does not then recite Birkat Ha’mazon. If he wishes to satisfy all opinions, he should eat more bread and then recite Birkat Ha’mazon. According to the strict Halacha, however, once he recited Al Ha’mihya he is no longer required to recite Birkat Ha’mazon.
Needless to say, this applies only "Be’di’abad" – after the fact, if one made the mistake of reciting Al Ha’mihya. Otherwise, one who ate bread must certainly recite Birkat Ha’mazon.
Summary: If one ate a quantity of bread requiring Birkat Ha’mazon, but mistakenly recited Al Ha’mihya, instead, he does not then to recite Birkat Ha’mazon.
