In Birkat HaMazon, the one who is saying Birkat HaMazon answers amen to his own bracha during the third bracha, Boneh Yerushalayim, as required by the text.
Why is it that here we say amen to our own bracha? If it is because it is the end of a "group of brachot" why would we not say amen to our own brachot in other instances, for example the 19th bracha in Shmoneh Esrei?
If someone is listening to the one saying Birkat HaMazon, when would her or she answer amen to the bracha of Boneh Yerushalayim; would it be before or after the one saying it says his own amen?
1) The Shulchan Aruch and the Rema both hold that we say Amen at the end the third bracha of the Birkhat HaMazon ("Boneh Yerushalayim") because that point is the end of the d'oraita Birkhat HaMazon, and the entire fourth bracha is mi derabanan. Nusach Sephardi / Edot HaMizrach is stringent to say amen softly so as not to lessen the importance of the fourth bracha, Ashkenazim are generally lenient in this regard. B'eH Bli nader I will post the exact siman and perek tomorrow.
2) someome hearing the Birkhat HaMazon and not saying it for himself would say amen after the one saying it reflecting the former custom of one person saying the bracha for the table and everyone else being exempted