1. Introduction

Since the 1980s, second language acquisition has received widespread attention. The popularization of multilingual phenomena has led researchers to pay attention to the complexity of non-native language acquisition. In the past 15 years, research on third language acquisition has gradually become independent of second language acquisition research, and theoretical and experimental methods have been increasingly explored. This article will systematically review the models and theories of trilingual acquisition, and explore important issues and future development directions through empirical research.

2. Three language acquisition models and experimental methods within the linguistic framework

We conducted an in-depth exploration of the theoretical models and experimental methods of trilingual acquisition, mainly based on the minimalist framework of formal linguistics, and discussed the acquisition mode of trilingual morphology syntax. Emphasis was placed on the correlation between language acquisition and human innate biological characteristics, as well as the role of "universal syntax" in trilingual acquisition. The definition of the research scope involves three key words: "third", "language", and "acquisition", including the number and order of languages that learners have already learned, as well as the scenarios and stages of language acquisition.

3. Research on the Acquisition of Chinese as a Third Language

In depth exploration of the acquisition research of Chinese as a third language, it is pointed out that the research on third language acquisition started relatively late, mainly based on Indo European languages, while there is relatively little research on East Asian languages as acquisition target languages. By analyzing 125 studies on trilingual syntactic acquisition, it was found that only 11 targeted East Asian languages, with 6 focusing on trilingual Chinese. Clements u0026 Dom í nguez (2018) tested the "surgical knife model" through a case study of native English speakers learning Chinese and found that participants would transfer syntactic structures from one or two languages to three languages, emphasizing the importance of structural similarity in transfer. Guo U0026 Yuan's (2020-2023a, b) study examined the early and later development of Chinese trilingual acquisition through online and offline tests, with a particular focus on the use of modal particles at the end of sentences. It was found that there were significant differences in performance between trilingual learners and second language learners, and the transfer source was directly influenced by structural similarity. Xiang U0026 Yuan (2020) supplemented Guo U0026 Yuan's research by examining the acquisition of fixed/indefinite finger usage in Mandarin through self paced reading. It was found that Cantonese dialect had no effect on Mandarin in the early stages of trilingual acquisition, and the influencing factors of suggestive transfer may be more complex. These research results are of great significance for understanding the process of trilingual acquisition, but they also point out the limitations of existing research, such as the possibility that the choice of language combinations may affect the judgment of transfer sources. Future research needs to examine more structures under stricter variable control to comprehensively understand trilingual development.

4. Conclusion

Summarized the current status and future directions of trilingual acquisition research, emphasizing the differences between trilingual acquisition and second language acquisition, including the influence of internal and external variables. Six key questions were raised, including the role of universal grammar, the endpoint of trilingual acquisition, experimental methods for distinguishing trilingual syntactic knowledge, trilingual online processing modes, the applicability of trilingual morphological syntactic acquisition models, and the impact of trilingual input. Call on more scholars to participate in research on trilingual acquisition, especially the acquisition of Chinese as a third language, and suggest that trilingual acquisition theories under the framework of formal linguistics be exchanged and referenced with other fields such as psycholinguistics, neurolinguistics, multilingual education, etc., in order to promote a deeper understanding of human language acquisition phenomena.