This study found that the cognitive constructions of teacher participants with regard to teaching relative clauses were influenced by their previous teacher training. However, their teaching cognitions were mostly shaped by their personal practical knowledge, their educational and professional experiences as language learners and teachers, on-going classroom experiences, and specifically, the unique socio-educational contexts in the ESL teaching environment.This study evaluated the cognition of four ESL teachers' instructional decisions on grammar teaching in a context where relative clauses are taught by online corpus-based materials. The results indicate that these teachers, despite the differences in their cultures, maintained a similar methodology in teaching relative clauses, It was shown that there is no definite methodology for teaching grammar online and that the teaching can first reflect an inductive approach and spontaneously transfer into a deductive approach. Given the specific context of teaching grammar to adults in an online English class setting, the teachers did not recognise the inadequacy of employing deductive approaches. From the aspect of the learners, the teacher participants' grammar teaching practices could consistently be described as some principles that were incorporated by less grammar rules to be learned, simple explanations to be understood, more examples to be noticed, and more activities to be practiced. The study of Borg (1999a) supports the present study, which focuses on grammar pedagogical contexts, improving 'our current understandings of formal instruction by shedding light not only on what teachers do, but also providing insight into the cognitive bases of these practices' (Borg, 1999a: 28-29). Hence, raising implications of language teacher education and its research, rather than language teacher cognition research, is targeted at suggesting the best language teaching methodology.