We studied the relations between English as a foreign language teachers' grammar knowledge and their in-action mental models (MMs) of children's minds and learning. The grammar knowledge we examined was English wh-constructions. A total of 74 teachers completed an assessment task and were classified to have deep, intermediate or shallow knowledge. Ten teachers (five with deep and five with shallow level) were videotaped teaching wh-questions. The data were analysed qualitatively to determine the teachers' MM, and then quantitatively to test similarities/differences in their MM's expression. The findings revealed an identical MM among all teachers that is expressed differently as a function of knowledge level.