This paper introduces a cognitive tutor designed for second language grammar instruction. The tutor adopted Corbett and Anderson’s (1995) Bayesian knowledge tracing model and provided adaptive training on the English article system. We followed the Competition Model (MacWhinney, 1997) and understood the article system as a galaxy of cues determining article usage on the basis of form-function mapping. Cues are in competition during language acquisition; hence cue contrast is predicted to be an effective instructional method. Seventy-eight students were randomly assigned to four article training conditions (to learn 33 cues) and a control condition (to write essays). We found that article-training groups significantly outperformed the control group in an immediate posttest and a delayed posttest. Specifically, our result also suggested that there was a significant interaction between cue contrast and cue type (definite vs. indefinite). Cue contrast promoted more learning on the indefinite cues (more difficult for learners). Knowledge tracing did not demonstrate such an interactional effect with cue types. Instead, it boosted the instructional effect promoted by cue contrast.