Commonly, low completion rates in massive open online courses have called into question the quality of their learning materials and instruction. This paper attempts to identify crucial factors of engagement and retention in language massive open online courses (LMOOCs) in the context of the open online program of Icelandic Online, a self-guided course for second language learners of Icelandic. The study seeks to explore the impact of factors associated with the course's instructional design on engagement and retention and reveal crucial determinants of attrition. The study depends on survey and tracking data from 400 learners and qualitative data from 62 informants in one course. It builds on previous studies on student engagement and retention in LMOOCs (Friðriksdóttir, 2018, 2019). The present study identified six content-specific factors that the majority of participants considered to be important for their motivation. Some factors, such as gradual and scaffolded presentation of input, had a positive impact on retention. Furthermore, statements from learners in the study who disengaged before completing show that non-course-related factors, such as time constraints, affect LMOOC retention. The study provides a new framework for how to promote student engagement and suggests specific strategies for other LMOOC developers.