Since the global COVID-19 pandemic began, online learning has gained increasing importance as learners are socially isolated by physical and psychological threats, and have to face the epidemic and take preventive measures to ensure non-stop learning. Based on socially situated cognition theory, this study focused on exploring the relevance of online learning ineffectiveness (OLI) predicted by self-regulated learning (SRL) in different phases of learning (preparation, performance, and

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... ction) and its interaction with mind-unwandered during the COVID-19 pandemic. The subjects of the study were senior general high and technical high school students. After completing the online questionnaire, the PLS-SEM method of the structural equation model was used to analyze the data. Results demonstrated that self-regulation in two phases of preparation (i.e., cognitive strategy and emotional adjustment) and performance (i.e., mission strategy and environmental adjustment) in SRL are positively related to mind-unwandered in online learning. Moreover, mind-unwandered in online learning was positively related to the self-reflection phase (i.e., time management and help-seeking) of SRL. Additionally, self-reflection of SRL was negatively related to online learning ineffectiveness. PLS assessments found that the preparation and performance sub-constructs of SRL were negatively related to online learning ineffectiveness mediated by mind-unwandered and self-reflection of SRL. The results suggest that teachers can enhance their students' self-regulation in online learning, and assist them in being more mind-unwandered in online learning.