Two approaches to grammar instruction are often discussed in the ESL literature: direct explicit grammar instruction (DEGI) (deduction) and indirect explicit grammar instruction (IEGI) (induction). This study aims to explore the effects of indirect explicit grammar instruction on EFL learners' mastery of English tenses. Ninety-four eleventh-graders were conveniently selected and randomly assigned into either the experimental group (EG) or the control group (CG). A pre-post tests design was used to collect the data. Before and after the treatment, the following tests were administered: rule analysis, grammar, and speaking. A delayed written test was given to both groups to assess students' retention of structure acquired; in addition, a questionnaire was provided to the EG to investigate their perception on the treatment. The results indicated that the EG significantly outperformed the CG in the analysis of grammar rules and the oral proficiency, except for the use of grammar structures in a pre-defined context. Convincingly, there was a positive correlation between the grammar rules and their subsequent use. This validates the cause and effect of grammar rules' acquisition and the use of them in receptive and productive stages. Also, the EG had favorable attitudes towards the instruction. This study may provide practical implications and techniques for improving EFL students' grammar performance in high schools in Vietnam.