This study provides new evidence concerning the pattern of acquisition of free and bound morphemes in Italian, based on the speech of 59 children recorded through a cross-sectional method. We found that inflectional morphology is mastered before free-standing morphology. Despite the great variety of verb inflections, the analyses showed that children partially master present indicative from early productions. Although free-standing morphemes are used correctly, they are optionally omitted. Here we have explored the use and omission of articles, clitics, the copula, and auxiliaries and have shown that omission is subject to certain constraints. Articles are mainly omitted from the root of the clause, much as null subjects, because from this position the requirement of clausal identification is voided. A higher omission of accusative clitics than dative clitics was observed that has also been explained in terms of the uniqueness checking constraint: accusative, but not dative clitics need to check the D feature twice, because the former, but not the latter, trigger past particle agreement. The uniqueness checking constraint has been adopted to explain the higher omission of auxiliaries with respect to the copula: the former, but not the latter, have to check the T feature twice. Together, these findings suggest that children omit, but in principled ways.