This research used a tutoring scenario to examine the spontaneous use of knowledge for teaching, which was measured from four perspectives: assessment, explanation, comprehension checking, and strategy instruction. The objective was: (1) to investigate whether student teachers could spontaneously utilise adequate knowledge for teaching, and (2) to compare the responses described by student teachers in a mathematical educational course to those in other educational courses to examine the influence of subject proficiency on the spontaneous use of knowledge for teaching. In two studies, participants described how they would respond to a hypothetical teaching situation in which a young pupil incorrectly solved a fraction problem. In their descriptions, nearly half of the participants failed to mention effective assessment or explanation techniques; a large majority did not mention checking pupil comprehension or providing instruction on learning strategies. No differences were found between participants who majored in mathematical education and those who did not. The study revealed the weaknesses of student teachers from a new perspective: a lack of spontaneity in using knowledge for teaching, which was seen even in students enrolled in a mathematical education course.