The input hypothesis is a hypothesis in second language acquisition developed by Stephen Krashen, which states that a language learner acquires the most benefit from receiving linguistic input that is just beyond his or her current interlanguage, or level of grammatical understanding。 This type of input is recognized as comprehensible input or "i+1," where "i" refers to the learner's interlanguage。 According to Krashen, comprehensible input is more likely to be gained from interacting with another speaker of the language。 The acquired system or acquisition refers to the subconscious process identical in all important ways to the process children utilize in acquiring their first language"。 While the learned system or learning refers to the conscious process" for those results in "knowing about" language (Krashen 1) 

3。1 Comprehensibility

This hypothesis is Krashen's explanation of how second language acquisition takes place。 Krashen claims that human beings acquire language in only one way---by understanding, or receiving "Comprehensible Input"。 "Comprehensible input" refers to those language materials "a little bit beyond" the learner's current level。 It is formulized by Krashen as "i+1”。"I" refers to learner's current level of a second language。 "i+1”indicates the next stage in the learner's language development along the natural order。 "1" refers to the gap between the current level and the next stage。 In a sense only when the language input is absorbed or internalized by the learner, acquisition can too easy。 If the input is take place。 The input must be neither too difficult to understand nor too difficult, the learner is too easy, the learner will learn nothing。 The little beyond the learner's present capacity will not pay attention to the input, and if it most useful form of input should be just a little beyond the learner's present capacity。 论文网

Krashen believes that the acquisition of second language depends on a large number of languages，but these language inputs must be valid。 Krashen with particular emphasis on language acquisition is generated by understanding the information, which also is that learners must be able to understand the input of the language material, and once these materials are too complex, the learners will focus on Language form, and cannot focus on the meaning of language communication。 Therefore, Krashen believes that language input must be effective。

According to Krashen, the Input Hypothesis simply states that "the only way that humans acquire language is by getting understanding messages or by receiving comprehensible input" (Krashen 2)。 So the Input Hypothesis shows that how Understandable language input as the only way to acquire language, then if the learner cannot understand the language input information, cannot learn the language。 "Comprehensibility" refers to the understanding of meaning, not the form of understanding。 Learners can hear or read the understandable language material。 The difficulty of these materials should be slightly higher than the learner's existing language level。   

3。2 Interesting and Relevant

Only a large number of language inputs have language acquisition, that is, a large number of listening or reading, or both。 In this process, learners will naturally absorb many useful language materials and language knowledge; otherwise, language acquisition is nothing but empty words。 And learning usually takes place in formal environments, while acquisition can take place without learning in informal environments (Krashen, 20)。

According to the affective filter hypothesis, certain emotions, such as anxiety, self-doubt, and mere boredom interfere with the process of acquiring a second language。 They function as a filter between the speaker and the listener that reduces the amount of language input the listener is able to understand。 These negative emotions prevent efficient processing of the language input。 The hypothesis further states that the blockage can be reduced by sparking interest, providing low-anxiety environments, and bolstering the learner's self-esteem。