The starting point of this paper is that at the base of any constructivist didactic approach there are three basic ideas about conceptual understanding (CU): a) CU as a result of the student's cognitive effort; b) CU is related to the student's preconceptions; c) metacognition determines CU. Despite the multitude of constructivist approaches, such as project-based learning, problem-based learning, inquiry-based learning or collaborative learning, most students are not included in CU. The explanation would be that school teachers, although they have theoretical knowledge of these methods, still apply conventional or frontal teaching. Our constructivist teaching model assumes that any type of constructivist lesson contains at least two basic elements: i) the teacher's guidance of the students' research actions (carried out in groups) and ii) formation of abilities to apply the newly acquired knowledge. We emphasize that any lesson is truly constructivist if it contains the elements of the 5E model: engagement, exploration, explanation, elaboration and evaluation. To these we add other two mandatory moments: the student's assumption of the cognitive goals of the lesson and the formation of intrinsic motivation. For intrinsic motivation we will rely on the learner's internal resources: prior knowledge and conceptions, active engagement, critical thinking and change of perspective, interaction and collaboration, metacognition and self-regulation of learning, building personal conceptual understanding. Based on the example of the chapter “Mechanical work, power and mechanical energy” in 7th grade, the practical way how to form CU, procedural knowledge and attitudes is shown. The role of bidirectional student-teacher interaction in ensuring the permanent guidance of the students' research action (or scaffolding) is also presented.