The impact of corrective feedback explicitness on second language acquisition remains a critical area of inquiry, yet research on classroom-based learning of Mandarin tones – particularly challenging for first language (L1) English speakers – remains scarce. Furthermore, most feedback studies are conducted in laboratory settings, which may not reflect real-world second language (L2) classroom learning. The few classroom-based studies that exist often lack internal validity, such as failing to include a control group. Additionally, individual learner differences are rarely considered when investigating feedback effectiveness. To address these gaps, this study compares the effectiveness of two feedback types – recasts and metalinguistic feedback – in U.S. university students’ learning of Chinese tones within an online communicative classroom environment. It also examines whether feedback effects are modulated by learners’ explicit knowledge of vocabulary tone values and working memory capacities. Forty-eight novice learners of Chinese were assigned to three groups (recasts, metalinguistic feedback, control) and completed an online synchronous course comprising four 65–85-minute sessions over two weeks. Feedback effects were assessed through controlled (sentence reading) and spontaneous (picture description) oral production tasks administered before, immediately after, and two weeks post-treatment. Phonological short-term memory was evaluated via a nonword recall test, while executive working memory was measured with a listening span test. Results revealed that recasts produced larger and more sustainable gains than metalinguistic feedback, particularly in spontaneous tone use. While phonological short-term memory had minimal impact, executive working memory predicted pretreatment tone accuracy and enhanced the effects of recasts but was negatively associated with the utility of metalinguistic feedback. Vocabulary tone knowledge was linked to pretreatment tone accuracy; however, improvements in this knowledge resulting from instruction did not influence feedback effectiveness. This study highlights the efficacy of recasts in Mandarin tone learning, reinforcing the superiority of implicit over explicit metalinguistic instruction for similar L2 phonological targets.
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