This document discusses interlanguage grammar in second language acquisition. It begins by defining what an "error" means in second language acquisition versus dictionary definitions. It then discusses the interlanguage hypothesis, which posits that learners have a systematic, evolving mental grammar at each stage of second language acquisition. Learners' errors provide insights into their developing grammatical hypotheses. The document also discusses how the interlanguage hypothesis relates to theories about the influence of learners' first language and universal grammar on second language acquisition.