The document explores the complexities of second language acquisition (SLA) and presents a framework for theory building that includes various learner variables such as age, cognition, and affective factors. It discusses common hypotheses and myths in SLA, such as the role of explicit vs. implicit learning, and outlines criteria for developing a viable SLA theory based on past research. Key theorists, including Krashen and Long, offer insights into the processes of language acquisition and the importance of input and output in language learning.