"I am really tired."
Translation:Ich bin wirklich müde.
I'm wondering if this isn't an attempt by Duo to enforce a distinction that has ceased to mean much in most of the English-speaking world. In particular: "really tired" as "authentically/actually tired", versus "really tired" as "very tired". I'd venture that "really" is more often used in such situations in the second manner these days than the first. In particular, "ich bin sehr müde" would seem a perfectly reasonable translation of "I am really tired".