Learner corpus research seeks to describe and thereby better understand learner acquisition, thus informing better teaching practice and creating an important bond between corpus linguistics and second/foreign language (L2) research. While much research exists for the study of L2 English, there is little research for the study of L2 German. This study explores the implementation of a corpus-based writing course in German studies at Rhodes University in South Africa with students at third-year level who were learning to write longer texts that present and defend an opinion. In order to evaluate the success of the writing course in teaching specific academic collocations, a diachronic collection of learner writing was undertaken. Written assignments were used to create a learner corpus, RUDaF (Rhodes University Deutsch als Fremdsprache). The creation of a learner corpus of German before and during the writing course served as a reflective tool to aid in the evaluation and improvement of corpus-based teaching materials. This is exemplified through the students' use of three specific opinion-related everydayacademic German collocations before and after the writing course. The learner corpus revealed a marked improvement in the use of taught collocations before and during the writing course. These methods provide insights for other language educators who wish to implement corpus-based methods for improving writing in a second or foreign language, and for all educators wanting to assess improvements in student writing.