The main purpose of theories of second-language acquisition (SLA) is to shed light on how people who already know one language learn a second language. The field of second-language acquisition involves various contributions, such as linguistics, sociolinguistics, psychology, cognitive science, neuroscience, and education. These multiple fields in second-language acquisition can be grouped as four major research strands: (a) linguistic dimensions of SLA, (b) cognitive (but not linguistic) dimensions of SLA, (c) socio-cultural dimensions of SLA, and (d) instructional dimensions of SLA. While the orientation of each research strand is distinct, they are in common in that they can guide us to find helpful condition to facilitate successful language learning. Acknowledging the contributions of
