Types of hints to consider: a general description reminding them how the concept works, a description that shows the user what the question is asking for, a description that explains to them why their answer is wrong, or a description that eliminates another possible wrong answer (e.g., if you have a multiple choice question and the correct answer is A, tell them why C is incorrect, then cross it out so they can't select it in the future).
Given these levels, develop a model that estimates how well the user knows the concepts at each level. Using your model, develop basic heuristics that allows you to adapt the game level to something harder or something easier, depending on how the user is doing. Remember to clearly explain your model and heuristics in your report.