"I write the letter tomorrow."
Translation:Jag skriver brevet i morgon.
They reply I was given by an admin (on another question) is that the Swedish sentences are always the original sentence in the course, and that the English sentence should be possible to translate back into Swedish with the same grammar. So if the English translation was I'm going to write the letter tomorrow, the most common answer would probably be Jag kommer att skriva brevet imorgon, but this would not reflect what I think the lesson of the sentence is - to use the present tense in a future sense.
I think this is a ghost sentence, we've changed the main English translation to I am writing the letter tomorrow which seems to be a little better, and this version should never be shown, but obviously it still was. There are a few sentences like that in the course, that have been removed but are still shown. They seem to be very hard for Duo to fix.
JoelWibron is right about the general idea behind why we don't want to translate it into the future tense even though that's how it would be most likely to be said in English.
In fact I write the letter tomorrow is not even an accepted translation into English at the moment (and the sentence hasn't been changed for a month), but there are 8 error reports that says it should be, so maybe I should add it back in.