Using deadjectival psych verbs with -garu in Japanese, this study shows that agglutinative complex predicate formation is done by recursive application of Merge to roots and functional heads. This process creates a layered syntactic structure, with each layer providing the computational system with (i) specific semantic features, (ii) arguments, and (iii) phonetic form (PF) exponents at conceptual–intentional (CI)/sensory motor (SM) Interfaces. The whole amalgam of the root and the functional heads is interpreted as a “word” at PF. Following the general architecture of Distributed Morphology, I will show that the morpheme that derives deadjectival verbs -garu is underlyingly -k-ar-u (k-Copula-T), where k is “little” v that originates in the verbal root k-o “come” and ar- is a copula. They are now grammaticalized functional heads that extend adjectival roots. Crucially, this k is homophonous with “little” a, which makes -garu and the adjective-deriving morpheme -karu (k-Copula-T) parallel. k is voiced in -garu due to a structurally conditioned assimilation rule (Embick 2013). This analysis reveals the mechanisms of agglutinative predicate formation in a precise and detailed manner. Similarly, it gives natural solutions to some of the long-standing problems including how adjectives modify N such as utukusii dansaa “beautiful dancer,” which is ambiguous between attributive modification and a relative clause.