The Natural Order hypothesis posits that there is a "natural order" that is predictable when it comes to acquiring grammatical structures. The Input hypothesis is completely in relation to the Acquisition hypothesis and it is especially vital to the understanding of how one learns a second language. Krashen (1997) believes that "if a learner is at a stage 'i', then acquisition takes place when he/she is exposed to 'Comprehensible Input' that belongs to level 'i' plus 1" (1997). This means that if students are at different levels, they can learn, essentially, from those who are more proficient.

The fifth and final hypothesis, Affective Filter, has to do with Krashen's belief that a number of "affective variables" play a role, though not a fundamental one, in second language acquisition. Some of these variables, according to Krashen, could be positive self-image, confidence or lack of confidence, low or high motivation, etc. The...
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