| This study tested the unique checking constraint hypothesis and the usage-based approach concerning why young children produced tense and agreement morphemes variably via three experiments. Experiment 1 investigated whether subject types influenced the production accuracy of auxiliary 'is' in three-year-olds through an elicited production task. The rate of use of auxiliary 'is' increased as children's tense productivity increased, but the pattern was different for each subject type. The rate of use increased more rapidly with tense productivity for lexical NP subjects than it did for pronominal subjects.;Experiment 2 further examined the role of subject types, predicate types, and predicate word frequency on the use of copula 'is' in three-year-olds. The production accuracy of copula 'is' was higher with nominal predicates than with permanent- or temporary-adjectival predicates, followed by locative predicates. Children also produced copula 'is' more accurately with low-frequency predicate words than with high-frequency predicate words. Moreover, the effect of subject types on the use of copula 'is' varied with children's tense productivity and predicate types. For sentences with nominal, permanent-adjectival, or temporary-adjectival predicates, children with lower tense productivity used copula 'is' more accurately with lexical subjects than with pronominal subjects in. This pattern, however, reversed in children with higher tense productivity.;Experiment 3 extended Experiment 1 by exploring the degree of abstractness of representations of auxiliary BE via a structural priming task. The production accuracy of auxiliary 'is' in three-year-olds increased above the baseline when the prime-target pair shared the same structure and subject + auxiliary 'is ' combinations, but not when the prime-target pair only shared the same structure. However, the production accuracy of auxiliary 'are' did not change with prime types. |
