Examining changes over time in a child’s syntactic repertoire provides insight into what exactly children are acquiring and how they acquire it. Case studies are presented to illustrate three general points: (i) children are highly conservative learners; (ii) change in the child’s syntax consistently takes the form of adding new structural options (rather than replacing old ones); and (iii) superficially unrelated syntactic forms may be systematically interconnected, in the sense that children acquire them as a set. Together these properties strongly suggest that the information being acquired is parametric, and that choices take the form of ‘subset’ parameters, where adopting the non-default value simply expands the set of 〈form, meaning〉 pairs permitted. One implementation, the Constructive Parameter Hypothesis, posits that non-default values add new structure-building operations; CPH is shown to make distinctive, testable predictions.

This study investigates the syntactic position of the nominative subject in child Japanese through CHILDES. We present two pieces of evidence for licensing the relationship between T and the nominative subject. The first piece of evidence deals with nominative subjects in the right-dislocated position. Dansako (2020) argues that case particle errors of subjects in Japanese are not attested in children’s right-dislocated sentences, namely, (O)VS order, in contrast with ones in canonical word order. The subject in the right-dislocated position is correctly nominative-marked. In many analyses of this construction (e.g., Tanaka (2001)), noncanonical order is derived via the movement of dislocated elements from canonical order. If this is the case, children’s dislocated subject should at least go through Spec-TP, resulting in error-free case markers attached to the subject. The second piece of evidence is children’s obedience to the restriction of subject interpretation. In Japanese, the nominative subject in the conditional clause cannot relate to the empty subject in the matrix clause over the clause boundary. Dansako (2022b) demonstrates that child grammar can perform the interpretation of the subject as required by the TP-related restriction. Our results suggest that nominative case is assigned within TP projection even in child grammar.

This study seeks to predict and verify the developmental process of Japanese benefactives, -teageru, -tekureru, -temorau based on Lexical Mapping Hypothesis (LMH), which is one of the hypotheses of Processability Theory (PT). The -teageru and -tekureru constructions have the same mapping structures according to the LMH analysis of benefactives. LMH also predicts that compared to -teageru and -tekureru, -temorau has a complex mapping structure, which makes the processing difficult for Japanese language learners. Utilizing the language corpus of 100 subjects in I-JAS, the study verified the developmental sequence of -teageru, -tekureru, -temorau. The results from the Implicational Scaling, which PT adopts as a distributional analysis, show that implicational relatives exist in the developmental sequence of benefactives among the learners of Japanese as [-teageru·-tekureru>-temorau] (C_scal=.89).