This document summarizes schools of thought in second language learning from the 1940s to 2000s. It describes how behaviorism and structural linguistics were dominant in the 1940s-1950s, focusing only on observable behaviors and language units. In the 1960s, cognitive psychology and transformational-generative linguistics emerged, considering mental processes, meaning, and innate language abilities. Finally, constructivism developed in the 1980s-2000s, emphasizing learners' active role in constructing understanding through individual cognitive processes and social interaction. The document concludes that no single theory can fully explain language learning.