Krashen’s Input hypothesis, which forms an important part of Stephen Krashen's theory of second language acquisition (1985), states that students improve and progress following a “natural order” in which they are constantly receiving input (“i”) from the target language at a higher level than that which corresponds to their current level of linguistic control. In other words, study material should always be “i + 1”.
 See also
- Acquisition-Learning hypothesis
- Monitor hypothesis
- Affective Filter hypothesis
- Natural Order hypothesis