The status of mental rules in language learning has long been subject to intense debate. Dual-mechanism accounts argue that regular forms are processed by explicit mental rules, whereas single-mechanism theories maintain that all forms are learned and processed in a single associative mechanism. The present study examines the learning of the morphology of German noun plurals by adult native English and Serbian speakers on the basis of rules, examples or both rules and examples. Results across these three experimental conditions suggest that the morphological patterns are learned more easily in the form of rules and thus, seem to be more easily captured by dual-mechanism than single-mechanism accounts. However, a closer examination of error patterns across the five plural patterns (-e, -n, -er, Ø, -s) revealed results contradicting dual-route theories and suggested the existence of two rule-mechanisms rather than one for learning regular inflection. Moreover, the n rule was the easiest one to be learned, although it is the s rule which is considered as a default rule in German.