Add to ORKGWithin the PT framework, this chapter represents an exploration in three new directions. First, we look at the development of a case system in learners. Despite PT’s fairly extensive empirical application to a variety of languages and acquisitional circumstances and populations (cf. § 1, ch. 1, this volume), surprisingly little attention has been given to the development of case systems, except for Baten’s work on German L2 (2011, 2013) and Artoni &Magnani’s on Russian L2 (2013) and in chapter 5, this volume. The system we will be looking at is that of Serbian, like Russian a heavily morphologised case-marking language (for an overview of Serbian from a typological point of view, cf. Corbett & Browne (2011). After a brief account of the cas...