"After the third hour we go to the forum."
Translation:Post tertiam horam ad forum imus.
Both tertiam horam and forum are in the accusative singular. They end with -m due to their declension's endings.
The reason they are accusative is because the prepositions post and ad take the accusative. Since post is used with tertiam horam, tertiam horam has to be in the accusative. Since ad is used with forum, forum has to be in the accusative.