The aim of the paper is to describe the acquisition of morphology (definite articles, clitic object pronouns and verb inflections) in Italian preschool children with typical development. First of all, grammar production will be examined, with a particular attention to the pattern of errors made by children; the causes will be attributed to the prosodic structure. Furthermore, the knowledge of grammatical function of the morphemes will be described. In this second phase, the dissociation between comprehension and production will be also analysed. Finally, the role of lexicon in grammar acquisition will be discussed.