A. The Domain Model
The domain model contains all the expert knowledge of a particular domain that needs to be taught by the system [2][11]. It details different concepts of the learning domain, their hierarchy, mutual interconnection, and how they relate to educational resources to be delivered [16][17]. Broadly speaking, there are three main ways how domain models are conceptualized [16]: the set/vector model, the hierarchy model and the network model. The most popular and widely used type of domain model is the network model. In this model, the concepts are arranged into a network and are interconnected by different types of links. The most common type of links are, amongst others, prerequisite links, “is-a” or “part-of” links, and remedial links [16]. Links in both hierarchy and network models are very useful in for improving the accuracy of the learner model. For example, in case the learner does not possess knowledge related to a certain concept (e.g., formation of the Simple Past Tense), the links help in providing the most likely remedial concepts (e.g., irregular verb forms). Conversely, if the learner possesses knowledge related to a certain concept (e.g., formation of the first conditional), the links help in identifying other concepts that the learner is likely to know (e.g., the Simple Present Tense).