This thesis examined the factors of L1, input frequency and emergent productivity in child L2 acquisition. This thesis is the first study to look at the interplay of L1 and usage factors in children learning a L2. The focus of the thesis was an investigation of these factors in the acquisition of article and auxiliary systems of English, which have been proven to be problematic areas for both L1 and L2 learners. While accounts of L1 transfer in L2 are better developed in generative theory, the roles of input frequency and emergent productivity are better developed in constructivist theory. The thesis assessed these two approaches against the data from L2 children from various L1 backgrounds. The children’s accuracy and error patterns with articles and auxiliaries were investigated. The main findings were as follows. L1 typology facilitated the acquisition of the structure of the NP and VP, but it only extended as far as the awareness of the presence of the functional morpheme (article or auxiliary). L1 transfer effects were observed only in the first 1.5 years of acquisition, which could be due to the unstable L1 knowledge in child L2 learners. The use of articles and auxiliaries was also influenced by their input frequencies and distribution, as more frequent forms were supplied more accurately and were substituted for less frequent forms. Different forms of articles and auxiliaries emerged separately and followed different paths of development. It was argued that they were acquired piecemeal and that productivity with these forms emerged gradually. It was concluded that constructionist theories were better supported by the data, since the findings on input frequency and productivity were not compatible with the generative approach, and L1 transfer was incorporated into the constructionist approach to account for the findings. It was argued that by the onset of acquisition, child L2 learners had established constructions in their L1 that were abstract enough to be transferred to L2 and did not rely on lexically specific information. As all children learned specific morphological forms of L2 piecemeal, in doing so they demonstrated input effects that held across all L1 backgrounds.

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This study tested the unique checking constraint hypothesis and the usage-based account 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 via an elicited production task. Overall, 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. 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. In contrast, children with higher tense productivity produced copula 'is' more accurately with pronominal subjects than with lexical subjects. 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. These experiments suggest that young children have only lexically-specific representations of auxiliary BE. Frequency, rather than structural properties, of sentence elements influenced the production accuracy of auxiliary and copula 'is' in young children. These findings support the usage-based approach that young children use tense and agreement morphemes variably because they have not yet learned adult-like abstract representations and use highly frequent/ lexically-specific constructions for the production of these morphemes.

The present diploma thesis focuses on the unconscious code-switching between second and third language. The main objective of this thesis is to deal with principles of unconscious code-switching, describe and categorize actual appearances of unconscious code-switching between second and third language. After introducing issues, the second chapter sums up the way of second and third language acquisition in terms of bilingualism and multilingualism. The third chapter presents a definition and theoretic bases of code-switching. The fourth chapter concerns the analysis of the reason for code- switching in terms of usage-based and psycholinguistic approaches. The fifth chapter presents a categorization of code-switching from syntactic and pragmatic perspectives. The sixth chapter concerns the analysis of cases of code-switching from recorded interviews and the explanation of possible factors causing unconscious code-switching. Key words: Unconscious code-switching, bilingualism, multilingualism, usage-based approach, psycholinguistic approach, function words, acquisition, second language, third language, WIPP