Stephen Krashen developed a theory of second language acquisition consisting of 5 main hypotheses: 1) Acquisition-Learning hypothesis distinguishes acquired implicit knowledge from learned explicit knowledge. 2) Natural Order hypothesis states language rules are acquired in a predictable order. 3) Monitor hypothesis describes how learned rules are used to edit language. 4) Input hypothesis emphasizes comprehensible input just beyond a learner's level. 5) Affective Filter hypothesis explains how affective variables like motivation and anxiety can help or hinder acquisition. Krashen's theory mapped out a "natural approach" to teaching language that focused on acquiring language through meaningful interaction and input at an appropriate level.