494. Forgetting to Bentch

Brachos 4:2

If a person is in doubt as to whether or not he recited HaMotzi, he does not repeat the bracha because (unlike bentching) this bracha is not required by the Torah. One who forgot to recite HaMotzi should do so if he remembered before completing his meal. If he remembered after finishing his meal, he does not recite the bracha.

Brachos 4:3

If a person was eating in one house, then he interrupted his meal and went to another house, or if someone came to his door and he went outside to speak to him, when he returns, he must bentch for what he ate in the first place, recite HaMotzi again because he changed places, and then he may finish his meal.