The production of grammatical markers takes a long time to master. Even when students know the rules, they do not systematically apply them. However, few studies have demonstrated the efficacy of interventions to improve this competence, and no study has addressed the issue at the cognitive and motivational levels jointly. Our study demonstrates the effect of combining progressive treatment, based on cognitive cost and self-assessment, on grammatical spelling and on self-efficacy beliefs. Over 8 weekly 50-minute lessons, four groups of 18, 20, 18 and 21 ninth graders received, respectively, progressive treatment alone, coupled with self-assessment, coupled with feedback, or coupled with self-assessment and feedback. These intervention groups were compared with a control group of 36 students receiving standard spelling instruction. The interventions focussed on past participle inflections, which are particularly difficult to learn for French-speaking students. The results indicated that (a) students who received progressive treatment made more progress than the control group students in spelling past participle inflections, presenting a transfer and a first degree of automatisation of the rules. The results also indicated that (b) students who participated in progressive treatment with self-assessment improved even more on the spelling tests, including free text production, and in their self-efficacy beliefs, demonstrating a deeper automatisation of the grammatical rules combined with an increase in perceived efficacy. This research shows that learning benefits from instructional practices that provide both cognitive cost and motivational support.