"I run home."
Domus (here domum) is one of the lucky nouns that has the locative. Nouns with the locative lose the preposition for place from where and place to where (which is used here) constructions.
Place to where : Domum curro -> "I run home" vs. Ad villam curro -> "I run to the house".
Place from where : Domo curro -> "I run from home" vs. A villa curro -> "I run from the house".
Place where (the locative is used for domus) : Domi curro -> "I run at home" vs. In villa curro -> "I run in the house".
Thank you. I would give you ten lingots if it allowed me to. I have been told to just to think about the five cases to begin with but I am getting an idea of what locative must be. I am pretty much just relying on my understanding of German and Spanish at this point. It looks like they have included a good mix of cases in these early lessons.
But the locative of "domus" is "domi" (meaning "at home")
"domum" here is actually a slightly weird adverbial usage of the accusative case that means "homeward". This usage only occurs with a handful of nouns, such as "domus" and "rus" (The list of nouns that are used this way in the accusative is pretty much the same as the list of common nouns that have a locative case.)