"Either he is already sleeping, or he cannot hear us."
Translation:Buď už spí, anebo nás neslyší.
The course generally allows the "sensing" verbs -- like hearing in this sentence -- to be translated with or without "can" on the English side. So in this exercise, I would expect both "cannot" and "does not" hear to be considered.
I don't recall any exercises where both have NOT been accepted for the sensing verbs, but (1) I haven't encountered every sentence in the course and (2) it's possible that the "dual use" decision hadn't yet been made when the course was first released.
"Cannot" je zkratka "can not"? Zatím jsem se v kurzu angličtiny s tímto výrazem nesetkala.