Stephen Krashen is professor emeritus at the University of Southern California, moving from the linguistics department to the faculty of the School of Education in 1994. He is a linguist, educational researcher, and activist. Dr. Krashen has published more than 350 papers and books, contributing to the fields of SLA, bilingual education, and reading.
He is credited with introducing various influential concepts and terms in the study of second language acquisition, including the Acquisition-learning hypothesis, the Input Hypothesis, Monitor Theory, the Affective Filter, and the Natural Order Hypothesis.
The distinction between acquiring and learning was made by Stephen Krashen (1982) as part of his Monitor Theory. According to Krashen, the acquisition of a language is a natural process; whereas learning a language is a conscious one. In the former, the student needs to partake in natural communicative situations. In the latter, error correction is present, as is the study of grammatical rules isolated from natural language. Not all educators in second language agree to this distinction; however, the study of how a second language is learned/acquired is referred to as Second Language Acquisition or SLA.
Krashen SLA notions http://www.csuchico.edu/~gthurgood/470/014_Krashen.pdf
Academic reactions to Krashen
Ellis (1990, p.57): 'the lucidity, simplicity, and explanatory power of Krashen's theory'.
Lightbown (1984, p.246): a combination of 'a linguistic theory (through its "natural order" hypothesis), social psychological theory (through its "affective filter" hypothesis), psychological learning theory (through its acquisition-learning hypothesis), discourse analysis and sociolinguistic theory (through both the comprehensible input hypothesis and the "monitor" hypothesis)'.
Mitchell & Myles (1998, p.126): 'The concepts of 'understanding' and 'noticing a gap' are not clearly operationalised, or consistently proposed; it is not clear how the learner's present state of knowledge ('i') is to be characterised, or indeed whether the 'i+1' formulation is intended to apply to all aspects of language, from lexis to phonology and syntax.'
Gregg (1984, p.94): 'each of Krashen's hypotheses is marked by serious flaws: undefinable or ill-defined terms, unmotivated constructs, lack of empirical content and thus of falsifiability, lack of explanatory power'
McLaughlin (1987, p.56): 'Krashen's theory fails at every juncture ... Krashen has not defined his terms with enough precision, the empirical basis of the theory is weak, and the theory is not clear in its predictions)
Ellis (1985, p.266): the Monitor Model 'poses serious theoretical problems regarding the validity of the 'acquisition-learning' distinction, the operation of Monitoring, and the explanation of variability in language-learner language'