Four blessings: Hazan (food), Nodeh (land/exodus), Rachem (Jerusalem), Hatov V'hameitiv (God's goodness). The first three are Torah-level; the fourth was added rabbinically after Betar.
Question 2
What is zimmun?
Zimmun is the communal invitation: one person leads, others respond. Three men = basic zimmun. Ten = God's name is added. A quorum eating together creates a communal obligation.
Question 3
Why was the fourth blessing (Hatov V'hameitiv) added?
The fourth blessing was instituted after the fall of Betar, when both Roman permission to bury the dead was granted and the bodies miraculously didn't decompose. God is good even in darkness.
Question 4
What do you add to Birkat Hamazon on Shabbat?
Retzeh is added on Shabbat, Ya'aleh V'yavo on Rosh Chodesh and Yom Tov. These insertions connect the meal to the sanctity of the day.
Question 5
Can a zimmun be formed after people have finished eating and begun to disperse?
Zimmun must be established while the group is still together at the meal. Once people disperse, the communal obligation dissolves.