Previous studies have revealed that students' participation is a complex matter affected by pedagogical, environmental, and individual factors. However, there is still insufficient empirical evidence regarding how those factors work in shaping classroom participation in the online context. In the field of language education, moreover, it still remains unclear as to how social interactions among students and teachers may affect students' cognitive engagement. To further understand the complex relations between language students' online engagement and the influencing factors, the current study conducted in-depth interviews with 24 university students enrolled in three online English courses in an International university in China. Analyses and interpretation of the qualitative data were informed by the Community of Inquiry framework. The findings suggest that students' engagement in online classrooms is a situated process affected by 1) pedagogical practices and support from teachers, which determines students' cognitive presence and perceived teaching presence directly, and 2) students' perceived social presence, which was shaped by both group dynamics and the online environment. Notably, the physically isolated online environment seemed to have played an impeding role in some students' cognitive engagement, but a facilitating a role in some others’, as mediated by their preference for social presence or absence. Overall, our study highlights the importance of providing students with a constructive, supportive and interactive environment for online language teaching and learning.