Usage-based theories have influenced English grammar instruction by focusing on real-world language use. Yet, grammar teaching remains relatively unchanged by research findings. This study, therefore, investigated the effect of corpus based instruction on grade eleven students grammatical accuracy with application to writing skills and their perceptions. By employing all the requirements of pre-test post-test quasi-experimental design, participants were assigned into an experimental group learnt through corpus based instruction and a control group received traditional instruction. The investigation focused on modal auxiliary verbs of ability and past simple habitual expressions identified as problematic areas. The four-week intervention covered two weeks for modal verbs of ability and two weeks for past habitual expressions excluding homework assignments. Data were collected through written tests, questionnaires, and semi- structured interviews. Test and questionnaire data were analysed quantitatively, while semi- structured interview data were analysed qualitatively using thematic analysis. The Ethiopian Students Corpus and British National Corpus were compared for frequency distribution and error patterns of target constructions. Pre-test and post-test results showed significant improvements in both groups with the experimental group outperforming the control group. Regarding to frequency distribution and error patterns, the results showed that "can" was more common in the Ethiopian Students Corpus, while "could" was frequent in the BNC, with no use of "was/were able to" in the Ethiopian corpus. Overgeneralization and subject-verb agreement errors were reduced in the experimental group. Differences in past habitual expressions also found with common errors like omitting subjects, incorrect use of "used to," and misusing "would" for singular past events. Most students showed positive attitudes towards corpus based instruction. From the results, it could be deduced that the instruction improved their ability to apply grammar in real-life contexts particularly benefiting from concordance lines though some