Support is needed to promote problem-based learning (PBL) and to enhance critical thinking skills in discussion-based Internet forums. By advancing the capabilities of chat room and forum software, problem-based discussions for learning can be supported further in online learning environments. In this paper, the authors report on MALESAbrain, an intelligent learning tool. The model is built on the notions of threshold and knowledge weight from the discipline of machine learning. The model encourages learners to judge or critically evaluate the solutions posted by others before exploring further knowledge content. The system then sums up the judgment scores as its knowledge weight to pass the thresholds setup for ranking/arranging the learning issues. This constraint design, therefore, becomes a mechanism for critical thinking in a problem-based learning activity. MALESAbrain as a model helps transform the forum and chat room into rich learning discussion environments. (Contains 12 figures and 1 table.)