|Evidence for the systematic nature|
There has been much debate about exactly how language is learned, and many issues are still unresolved. There have been many theories of second-language acquisition that have been proposed, but none has been accepted as an overarching theory by all SLA researchers. Due to the interdisciplinary nature of the field of second-language acquisition, this is not expected to happen in the foreseeable future.
Sequences of acquisition
Download The error in the Second Language Acquisition [Pdf] disini
Download Native Language and foreign language acquisition [Pdf] disini
Evidence for the systematic nature of the stages of foreign language acquisition