  Automated text simplification, a technique useful for making text more
accessible to people such as children and emergent bilinguals, is often thought
of as a monolingual translation task from complex sentences to simplified
sentences using encoder-decoder models. This view fails to account for
elaborative simplification, where new information is added into the simplified
text. This paper proposes to view elaborative simplification through the lens
of the Question Under Discussion (QUD) framework, providing a robust way to
investigate what writers elaborate upon, how they elaborate, and how
elaborations fit into the discourse context by viewing elaborations as explicit
answers to implicit questions. We introduce ElabQUD, consisting of 1.3K
elaborations accompanied with implicit QUDs, to study these phenomena. We show
that explicitly modeling QUD (via question generation) not only provides
essential understanding of elaborative simplification and how the elaborations
connect with the rest of the discourse, but also substantially improves the
quality of elaboration generation.
