A study of Jordanian English-as-a-Second-Language learners examined their metalinguistic knowledge of 15 English grammatical concepts or categories via rule description and exemplification, and also explored the link between metalinguistic knowledge and proficiency in using those categories or their exponents in both formally and situationally-oriented contexts. Subjects were 233 grade 10 students. The 15 categories were presented to the students in four tasks: one eliciting metalinguistic knowledge; one eliciting descriptive metalinguistic knowledge; and two demanding formal and communicative use, respectively. The responses were sorted according to whether the first or second task was accomplished better, and analyzed for patterns in the third and fourth tasks. Results indicate a considerable awareness of both exemplary and descriptive metalinguistic knowledge, with somewhat more exemplary knowledge. The "exemplary knowledge" group was significantly more successful at using the categories both formally and situationally than the descriptive group. However, both were more successful at formal than communicative use of the categories. It is suggested that metalinguistic knowledge enhances language learning. Implications for classroom instruction are offered. Contains 30 references. (MSE)