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Stephen Krashen Professor emeritus at the University of Southern California is a highly acclaimed, controversial linguist, who has put forth a number of hypotheses on second language acquisition, particularly in bilingual education.

One of the more important ideas he puts forth is that in order for a student to learn, the input must be just slightly above a 'comprehensible' format. The theory contravenes that of 'sink-or-swim' educators who stipulate that through sheer exposure and repetition, students will acquire the target skills, in this case, target language. The cartoonish scenario of repeating a word more and more loudly to an uncomprehending listener is the opposite of Krashen's 'comprehensible input' premise.

Building on the idea of 'comprehensible input' for language learners, 'acquisition' is a sub conscious process. While it is happening you may not know it is happening. A key assertion in Krashen's work is that language 'acquisition' and 'learning' are two different processes. 'Acquisition' occurs in communicative situations in the 'real world'. 'Acquired' knowledge is readily available to communicate meaning. 'Learning' occurs through formal training (the classroom); it cannot be used to communicate 'meaning'. Instead, it monitors the proper grammatical use of 'acquired' language knowledge.

Krashen believes that language learners must build on their knowledge through processing language at a level slightly beyond their ability: 'current competence + 1' (i +1).[1] This is also known as instructional scaffolding.

Krashen also penned a piece with Tracy D. Terrell on the natural approach (1983), which is more or less a continuation of the comprehensible input theme with the added idea that affective filters or emotional barriers must be lowered in order for learning to take place.

Recognizing Krashen's contributions to linguistics he was characterized at M.I.T. as having made the most important contributions to linguistics in the last century.


  1. Krashen (1982), p. 21.


Krashen, Stephen (1982). Principles and practice in second language learning and acquisition, Oxford: Pergamon.

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