Most usage-based research emphasizes the importance of implicit, input-driven learning in naturalistic
environments, but recent studies have adopted usage-based grammatical descriptions for instructed learning in classrooms. These
descriptions are intended to draw learners’ deliberate attention to relevant usage patterns in the input and thereby support
intake. Most of these studies compare usage-based descriptions to other types of descriptions for their efficiency, while little
attention has been paid to the ways in which learners understand and apply such descriptions. This study examines what foreign
language learners understand of usage-based grammatical descriptions of target structures. In an experimental forced choice task,
Chinese learners of German received usage-based descriptions of case structures and then classified target instances in variable
contexts. A multivariate regression analysis indicated that choices were influenced by interactions of the type of description
with participants’ target-language proficiency and the semantic and lexical target contexts. This is discussed in terms of
noticing and category formation. This study argues that learners are able to use grammatical descriptions as some kind of
auxiliary model for recognizing and categorizing target patterns. The descriptions thus make learners aware of the mechanisms
underlying implicit learning and help them exploit these mechanisms for explicit learning.