This document summarizes key points from Chapter 9 of Brown's book on principles of language learning and teaching. It discusses cross-linguistic influence (CLI) and learner language. There are 6 categories of difficulty in the contrastive analysis hypothesis. CLI considers influence between all existing language systems, not just the first language. Factors determining CLI include typology, the second language, and proficiency level. Error analysis examines errors from all sources. Learner language develops through random, emergent, systemic, and stabilization stages. Variability in learner language depends on context. Fossilization occurs when incorrect forms become permanent. Feedback on errors should consider affective and cognitive aspects.