This study was conducted at a university based in KwaZulu-Natal, South Africa. The purpose of the study was to determine if the explicit instruction of selected grammar aspect, modal auxiliary verbs, improved students' ability to write English. The study was qualitative in nature and a case study design was adopted. The focus was in relation to a sample of 80 student teachers who were randomly selected in 2016 in the Faculty of Education. 40 participants were randomly assigned into experimental and control groups. For the experimental group, training lasted six weeks. Both groups were made to write similar essays and those essays were marked focusing on the students' ability to use modal auxiliary verbs. The study's findings revealed that the experimental group performed better than those in the control group in the use of modal auxiliary verbs. Based on the findings, the study recommended explicit grammar instruction in all the students' level of study in order to overcome the challenges they have in writing English. Thus, time should be created to ascertain that adequate explicit grammar lessons are offered to all pre-service teachers at the university.