<p>The education landscape is undergoing a paradigm shift with the increasing integration of Artificial Intelligence (AI) in instructional settings. This study addressed the evolving role of AI compared to human teacher assistants, explicitly focusing on language learners’ progress and retention of complex sentences. Complex sentences are crucial components of linguistic proficiency, and understanding how AI and human assistants contribute to their learning is imperative in the context of contemporary language education. This research investigated the efficacy of AI and human teacher assistants in facilitating language learners’ understanding and retention of complex sentence structures. An explanatory sequential mixed-methods approach comprising quantitative and qualitative phases was used. The quantitative phase employed a quasi-experimental design with a repeated measures approach, while the qualitative phase utilized a qualitative case study design. Participants were 90 pre-intermediate level language learners, selected through convenience and purposeful sampling. Data analysis involved statistical methods for quantitative data and thematic analysis for qualitative data, providing comprehensive insights into the research questions. The study revealed significant quantitative and qualitative differences between AI, human, and no teaching assistant conditions in language instruction. Quantitatively, AI-assisted learning led to the most substantial gains, with learners achieving a mean score of 24.23, outperforming those with human assistants (<i>M</i> = 20.26) and those with no assistance (<i>M</i> = 16.30). Qualitatively, AI was praised for its accessibility and personalized feedback but lacked emotional connection. Human assistants provided emotional support and contextualized explanations but had availability challenges. Findings have different theoretical and practical implications for teachers, language learners, and educational administrators.</p>