The German subjunctive mood (‘Konjunktiv’) is a grammatical feature that attests to the freedom of human beings to step out of the boundaries of their immediate situation and explore new possibilities in hypothetical scenarios. It therefore plays a pivotal role in advanced learner argumentative writing, but due to its highly complex nature regarding both form and usage, the ‘Konjunktiv’ challenges learners on virtually all levels of linguistic knowledge. This paper presents the first learner corpus-based study of this phenomenon with a two-pronged Contrastive Interlanguage Analysis approach: comparing learner data to native-speaker data and learner data at different levels of proficiency using the Corpus of Learner German (CLEG). The investigation seeks to explain the detected patterns of over-, under- and misuse and developmental paths by drawing on cognitive theories of second language acquisition.