This study examined the effectiveness of explicit instruction and feedback, focusing on degrees of prior knowledge of the personal "a" in intermediate level second language (L2) Spanish. On the pretest, participants (n = 58) completed grammaticality judgment and picture description tasks, which found that learners had different degrees of receptive and productive command of the structure but had not mastered it above 90% accuracy. Based on pretest scores, participants were split into two groups: high prior knowledge (some receptive and productive command) and low prior knowledge (some receptive command only). On the posttest, half of each group was given computer-administered explicit instruction and feedback on the personal a followed by the two tasks. The control groups completed only the tasks. Results indicated that both instructed groups improved more than their respective control groups after treatment; however, while six individuals from the uninstructed high knowledge group improved over time on both tasks, no one from the uninstructed low knowledge group improved. Awareness results help to explain this difference, since both high prior knowledge and instruction with feedback were associated with higher levels of awareness. These results reveal a link between prior knowledge, awareness and the usefulness of instruction and feedback.