SUMMARY
Background: The study of the initial mathematical knowledge of primary education teachers in training is considered important, since this knowledge influences that of the students. Objectives: to study the initial knowledge of numbers and geometry of the pre-service teachers. In addition, some of the mistakes they make are analysed. Design: to carry out the study, 20 questions released from the TIMSS tests were used, specifically, arithmetic (numbers) and geometry questions were selected. Setting and Participants the TIMSS-type test was applied to 97 first-year pre-service teachers. Data collection and analysis: This research is quantitative and the sample used in this investigation is a purposive sample, participants answered a questionnaire with questions the knowledge of mathematical content in numbers and geometry. Results: the results indicate that, in general, the students present greater difficulties in the geometry questions. Specifically, 36% of students fail geometry, while 14% fail in the case of numbers. In problem-type questions, which belong to the cognitive domains of application and reasoning, a high percentage of errors and blank answers are obtained. Finally, in the study of errors, the difficulties that students have with the decimal numbering system are shown. It is also observed that they present difficulties in the matter of geometric measurements (areas, perimeters and volumes). Conclusions: In this research, on the one hand, weaknesses that can be reinforced by means of the mathematics subjects of the Degree have been detected. On the other hand, errors about the decimal numbering system should be a warning indicator to try to improve its understanding during the Primary Education stage, which is when it is introduced.