"Two pictures are in the building."
Translation:Duae picturae sunt in aedificio.
"Aaedificium" is 2nd declension neuter. "Aedificia" is the plural nominative. Here, we need "aedificio", the singular ablative.
The two forms are very different! For one thing, aedificia is PLURAL, "buildings" (in the nominative and accusative cases); whereas aedificiō is SINGULAR, "building" when it's ablative case, as it is, here, because of the preposition that means IN (= in + abl).
"In buildings" would be in aedificiīs , versus "in the building," in aedificiō .
"The building is large" = Aedificium est magnum . "The buildings are large" = _ Aedificia sunt magna_ .
We see the building = Aedificium vidēmus . We see the buildings = Aedificia vidēmus .
In other words, aedificium, aedificiī , n., is a 2nd declension neuter noun, and for neuter nouns, their nominative and accusative cases are identical; plus, in the plural, their nomin. and accus. cases end in -a.