As a negative consequence of COVID 19, almost all students felt obliged to leave the universities and use electronic devices to learn. Some universities encouraged faculty members to use Electronic teaching as an alternative to traditional classrooms. Despite a large number of studies on different forms of electronic learning, the effectiveness of synchronous and asynchronous E-learning in teaching grammar to EFL learners has been neither investigated nor compared appropriately. Two intact classes of undergraduate students of English as a Foreign Language (EFL) from Farhangiyan university were selected. and each received the treatment through either synchronous or asynchronous E-learning activities. The same syllabus consisting of dependent Clauses, Appostive, infinitives, and gerunds were taught to the two groups. EFL learners’ knowledge of grammar was measured by recognition and production tests. The two groups’ scores were submitted to repeated measures ANOVA tests. The findings showed that both modes of E-learning greatly influenced the language learners’ knowledge of grammar as measured by recognition and production tests. Results also showed that the interaction between electronic modes and grammar recognition and production tests was statistically significant. Therefore, it could be concluded that E-learning learning is useful, synchronous learning is more effective for improving the language learners’ production, and asynchronous learning is effective for improving the skills and subskills which require reflection and comprehension.