"The dirty boys do not go into the atrium."
Translation:Pueri sordidi in atrium non eunt.
Rather, it's "in atrium" if there is actual motion INTO the atrium.
It can only be "in atrio" if they are already LOCATED IN the atrium.
(In is a rare preposition--though it also happens with sub--which is constructed with both accusative and ablative, depending on the meaning you want: into the atrium = in + accus. / in the atrium = in + abl.)