The document discusses several key concepts in second language acquisition (SLA) theory, including Krashen's input hypothesis, Long's interaction hypothesis, Chomsky's views on language acquisition, and Krashen's acquisition/learning hypothesis. It also examines notions of comprehensible input, the affective filter hypothesis, the monitor hypothesis, interlanguage, and negotiation of meaning. Generalizations from SLA research are presented regarding how adults and children acquire a second language in systematic ways through practice, but not in a way that achieves full native-like mastery.