This paper explores the nature of the differences in the processing of morphologically regular and irregular forms in the brain. Verbs cannot be simply divided into regular and irregular in Russian–there are many inflextional classes that differ in defaultness, type frequency, and productivity. In the present functional magnetic resonance imaging study, we chose three verb classes that allow teasing these factors apart and asked 24 subjects to select verb forms agreeing with different pronouns. We combined measures for local brain activity and generalised psychophysiological interactions. We revealed that regularity effects are primarily driven by defaultness associated with more effective and automated processing in the left-lateralised fronto-temporal combinatorial brain network rather than by productivity or type frequency.