Why do we say "Hashem" instead of "Ado Nai" when that is instead of pronouncing the yud-hei-vav and hei?
The Meforshim on the Mishna at the bottom of Yoma 35b discuss it.
IIRC it's the Ritva who says that when we quote a Pasuk we say the Real Name as mentioned in the Pasuk, but when we tell a story we say Hashem.
Thus in the Yom Kippour Machzon during the Avoda section in Mussaf we recount how the Kohen Gadol says "Hashem" during his Viduy and then we quote a Pasuk mentioning Gɔd's name explicitly.
BTW: Twice in the Torah it says Hashem when referring to Gɔd:
But in any case, we never say the yud-hei-vav and hei as it's written, as the Gemara teaches us in Kidushin 71a and Pessachim 50a:
In Yoma 18b we find that even after the Cohen Gadol said the yud-hei-vav and hei name, people [miraculously] forgot it: