This hypothesis seeks to explain how second languages are acquired. In its most basic form, the input hypothesis argues that learners progress along the natural order only when they encounter second language input that is one step beyond where they are in the natural order. Therefore, if a learner is at step one from the above list, they will only proceed along the natural order when they encounter input that is at the second step.
The Affective Filter Hypothesis
This hypothesis describes external factors that can act as a filter that impedes acquisition. These factors include motivation, self-confidence, and anxiety. For example, if a learner has very low motivation, very low self-confidence, and a high level of anxiety, the affective filter comes into place and inhibits the learner from acquiring the new language. Students who are motivated, confident, and relaxed about learning the target language have much more success acquiring a second language than those who are trying to learn with the affective filter in place.
A Final Point on Grammar
According to second language acquisition theory, the role of grammar in language acquisition is useful only when the learner is interested in learning grammar. Otherwise, Krashen argues that studying grammar equates to language appreciation and does not positively influence language acquisition.
The Acquisition-Learning distinction is the most fundamental of the five hypotheses in Krashen's theory and the most widely known among linguists and language teachers. According to Krashen there are two independent systems of foreign language performance: 'the acquired system' and 'the learned system'. The 'acquired system' or 'acquisition' is the product of a subconscious process very similar to the process children undergo when they acquire their first language. It requires meaningful interaction in the target language - natural communication - in which speakers are concentrated not in the form of their utterances, but in the communicative act.
