Learner language, also called interlanguage, refers to the developing language system of second language learners. It is systematic but dynamic as learners receive more input and revise their hypotheses. Studying learner language helps teachers understand learner progress and errors. Learners form rules from intake and transfer from their first language can lead to errors, though it also aids learning. Errors reflect learners' current understanding and follow developmental sequences as interlanguage gradually approximates the target language.