"The letter is in the house."
Translation:La letero estas en la domo.
Because of the preposition. It only becomes domon with "en" if you indicate motion ("into"):
- Mi ĵetis la leteron en la domon. I threw the letter into the house.
preposition => nominative. Only prepositions like "en" which can mean or place or direction have "-n" (accusative) if they mean direction.
I don't know the grammatical rule on it, but the introductory material to the lesson stated that with "estas" you don't use the accusative. Probably for the reasons that others have stated. (By the way, with the COMPLETELY regular rules of Esperanto, the introductory material is of especial use! The lessons ALWAYS follow the pattern!)
That's true, but for different reasons: you don't use the accusative with what would seem a direct object (something after the verb, not followed by a preposition) with "esti" because it's a predicate subject (just like in German), here you don't use it because you have the preposition.
It's important to clarify that because the same pattern doesn't apply with other verbs (on the accusative) but it does so with the prepositions - you still shouldn't use the accusative after a preposition with them unless they fit the exceptions Andi and I have noted.