Introduction: Second language acquisition research aims to develop theories that predict and explain what learners can acquire. This study focuses on the acquisition of definite restrictive relative clauses by native speakers of Latakian Syrian Arabic. Objective:The objective of this study is to investigate the acquisition of definite restrictive relative clauses by learners of Latakian Syrian Arabic, contributing to our understanding of second language acquisition. Methods: A quasi-longitudinal design was employed in this study. Learners at different proficiency levels (elementary, lower intermediate, upper intermediate, and advanced) completed tasks including a grammaticality judgment task, a guided gap-filling task, and a translation task. Results: First language influence was observed at early stages of learning, particularly on some properties of relative clauses. However, not all properties showed this influence. Persistent influence of the first language was noted in later stages of learning, particularly on properties involving uninterpretable features. On the other hand, interpretable features seemed to be fully acquired. Conclusion: The findings suggest that while some aspects of definite restrictive relative clauses are influenced by the learners' first language, particularly those involving uninterpretable features, other aspects appear to be fully acquired. This sheds light on the process of second language acquisition and has implications for understanding the role of Universal Grammar (UG) in language acquisition. Keywords: UG, definiterestrictive relative clauses, proficiency, uninterpretable features