Abstract This paper aims to explain recent empirical findings on subjunctive II formation in the Bavarian dialects of Austria from both functional and formal perspectives. For this purpose, the explanatory power of the functional principles of natural morphology (NM) is compared with the formal framework of constructional morphology (CxM). It is argued that the two approaches complement each other. Thus, it is shown that the key concepts of NM (constructional iconicity, uniformity and transparency) can easily be adapted in terms of CxM. These adjustments are needed to explain the ongoing changes in subjunctive II formation in the Bavarian dialects of Austria. This is due to the well-documented shift towards the use of periphrastic constructions (with the täte- and würde-auxiliary) that are located at the interface between morphology and syntax.