An apparatus and a system to teach a user a subject based on his questions. The system allows the user to control his learning process, and helps to fill in gaps of misunderstanding in the subject. In one embodiment, the system, including a database, presents study materials on the subject to the user. After working on the presented materials, the user enters his question into the system, which generates an answer to the question, and presents it to him. Then the system compares the question with one or more questions previously entered by the user to determine his understanding level in the subject. Based on the determination, the system may present to the user appropriate study materials. The user typically asks more than one question, and the process of answering his question by the system repeats.