This experimental study, using a pretest-intervention-posttest design, aims to explore the effect of semi-flipped instruction on the grammatical competence and writing skill of basic users of English. These users were also considered to be low achievers among their classmates, based on the opinions of their teachers, the criteria of the institute as well as the Common European Framework of Reference (CEFR) for languages. The participants consisted of 53 Iranian students, who were assigned to two groups: the flipped and non-flipped. Each group received ten sessions of intervention. First, it was investigated whether there was any significant difference between the two groups in terms of their grammatical competence and writing skill. The flipped group significantly outperformed the non-flipped one. Next, it was examined whether the non-flipped intervention caused any significant change in the grammatical competence and writing skill of the non-flipped participants, and significant changes were found. The superiority of the flipped pedagogy can be attributed to the process of actively engaging the students in their learning activity in addition to including various techniques, such as collaborative writing, in-class teacher-learner interaction and negotiation as well as the video screencasting, because it is argued that the essential feature of flipped instruction is how the instructors best utilize in-class-time with the learners.