It is commonplace in the summer months for people to accept Shabbos early. There are several issues that arise in this situation.
When one davens Ma’ariv before nightfall he must repeat the recitation of the three parshiyos of Shema after nightfall. If one accepts Shabbos earlier, he must remember to repeat Shema after nightfall. One need not stop eating his Shabbos meal to say Shema, since he started eating at a time when he was permitted to do so. However, if one made early Shabbos and waited to start the meal until after nightfall, he would be obligated to recite Shema prior to eating.
There is a question that often arises when one makes early Shabbos. It is generally already night at the conclusion of the Friday night seudah; thus, one is obligated to recite Shema. There are now two mitzvos that one is obligated in – Birchas Hamazon and reciting Krias Shema. So, which one should he perform first?
The Shagas Aryeh, in siman 21, discusses this question at length. The order in which mitzvos are supposed to be performed is based on their frequency, tadir veshe’aino tadir, tadir kodem (the more frequent precedes the less frequent). However we need to clarify which is the more frequent one in this scenario. On the one hand, the mitzvah of Birchas Hamazon can be performed many times a day, in fact as often as one wishes to eat bread; whereas the mitzvah of Krias Shema applies only twice a day.
The Shagas Aryeh proves from a Gemara (Zevachim 90b) that the frequency of a mitzvah is not determined by how often one can perform it, but rather by how often one is obligated to perform it. Although there are times when one is obligated to eat bread and bentch, such as three times on Shabbos and twice on Yom Tov, the obligation to recite Shema is far more frequent. Even though once one eats bread, he is thereby obligated to bentch, it is not considered an obligation in this regard since he was not obligated to enter into that predicament.
The Shagas Aryeh concludes that one should first recite Krias Shema and then bentch, since the obligation to recite Krias Shema is more frequent.
I had thought to say that perhaps there is another component which needs to be addressed. Tosafos in Chullin 87a and the Rosh in Chullin (7:6) say that Birchas Hamazon is the completion of a meal. Perhaps there is room to say that since one was allowed to complete his meal (as he started prior to the time to say Shema) he should be allowed to recite Birchas Hamazon which is the completion of his meal. Clearly, the Shagas Aryeh did not opine that it is reason to recite Birchas Hamazon first.
Another common scenario occurs during the period of Sefiras Ha’Omer. If one was eating bread at night and had not yet counted Sefiras Ha’Omer, he will have two mitzvos that he will be obligated in – bentching and counting Sefirah. Once again, the dilemma arises: which mitzvah should precede the other?
The Shagas Aryeh, in siman 22, says that the correct order differs if it is Shabbos or Yom Tov, or if it is a weekday. On Shabbos and Yom Tov when there is an obligation to eat bread and bentch, the mitzvah of bentching should come first since the obligation to bentch is more frequent than the obligation to count Sefirah. But on a weekday when there is no obligation to eat bread, the obligation to count Sefirah is more frequent. Thus, one should count Sefirah first, and then bentch. Additionally, in this scenario, if one had not yet recited Krias Shema, its recitation would precede both Sefirah and bentching, since the obligation to recite Krias Shema is more frequent than both of those mitzvos.