Mes parents allaient au resto. - pas accepté, mais pourquoi?
Was wondering the same thing. Resto has been suggested for restaurant plenty of times.
"Mes parents allaient dans le restaurant." was NOT accepted. (Aptil 2020)
I guess that would require "... into the ... " in English.
Exactly. That would sound like they were entering the restaurant but for another reason than having a meal.
Then why is dans used for "we used to come to this restaurant often"
(It gave "Nous venions souvent dans ce restaurant")