Question: What’s the difference between birkas hamazon and shemone esray? Why does the Ramban brought in the Magen Avraham (192:1) hold that if you only know one paragraph (blessing) of birkas hamazon you can say that even if you don’t know the rest but Beur Halacha in siman 593 says in the name of the Nahar Shalom that if you know only one paragraph of Shemone esray then you can’t just say that one paragraph for the prayer of shemone esray?
A. The Beur Halacha also says that the Magen Avraham (593:2) who seems to argue by Shemone esray really hold that you do fulfill a Torah level mitzvah of praying even if you just say one paragraph of shemone esray but you don’t fulfill the mitzva of shemone esray where as you would fulfill the mitzvah of birkas hamazon if you only say one paragraph.
B. Though Shemone Esray is made of 18 different blessings it’s different than the 3 different blessings of Birkas hamazon.
Answer: Shemone esray the concept of asking mercy from Hashem and therefore it’s a prayers which is considered one unit so if you leave something out then the unit if prayer is lacking and doesn’t count but Birkas hamazon isn’t a prayer, it’s a bunch of blessings so one doesn’t effect the other according to the Ramban. The Magen Avraham does bring down the view of the Rif that all blessings of Birkas hamazon are connected so if you only know one then you can’t say Birkas hamazon at all.