This document summarizes theories of how humans learn languages. It discusses behaviorist theories from Pavlov and Skinner, cognitive theories from Ausubel about meaningful vs rote learning, and constructivist theories from Rogers about empowering learners. It also outlines eight types of language learning, including signal, stimulus-response, and concept learning. Factors like transfer, interference, overgeneralization, inductive/deductive reasoning, aptitude, intelligence and methods like audiolingualism and total physical response are summarized in relation to second language acquisition. The conclusion states that comprehensible input at the right level is key to language learning.