This study analyzes the mathematical knowledge of 40 preservice Chilean Early Childhood and Primary Education teachers when designing mathematical tasks on patterns, in the context of teaching early algebra. Based on the domains of the Mathematical Knowledge for Teaching (MKT) model, we have adopted a descriptive qualitative methodological approach that relies on the content analysis technique. The results show that pre-service teachers exhibit little mathematical knowledge in the description of the mathematical tasks they pose, addressing partial aspects of the subdomains of specialized content knowledge and knowledge of content and teaching. We conclude that training experiences should be given that allow students to further their acquisition of mathematical knowledge in order to achieve effective learning of the contents that early algebra promotes, such as patterns, in Early Childhood and Primary Education classrooms.