Krashen's Monitor Model, or Input Hypothesis proposed that the necessary and sufficient condition for second language acquisition was comprehensible input.  Learners needed only to hear samples of the target language that they could understand, but that were slightly ahead of their own current level of interlanguage production (i + 1) for acquisition to occur.

Krashen distinguished acquisition, a subconscious process, from learning, or conscious linguistic knowledge; only acquisition could promote spontaneous language use, and conscious learning was only useful for monitoring, or checking language output during written tasks.  Thus grammar instruction was unnecessary, since acquisition would proceed according to a natural order, and even harmful, since traditional formal grammar instruction hindered acquisition by placing learners under stress and raising their affective filters.

Many second language researchers have criticised the untestability of the majority of Krashen's hypotheses.  Where Communicative Language Teaching has been implemented and researched, most notably by Swain in Canadian immersion contexts, the approach has been found inefficient, and likely to promote fluency but not accuracy in second language production.

Read more: Krashen, S.D. (1982). Principles and Practice in Second language acquisition.