(1) Input hypothesis claims that “Human acquire language in only one way—by understanding messages or by receiving ‘comprehensible input’”. According to Krashen， input that is useful for L2 acquisition must be neither too difficult nor too easy to understand and ought to be tuned just right to learner's current level, represented as “i” . In the course of acquiring the L2， learners progress from one level to another. The next level is called “i +1”. For “i +1” to occur, the input has to be slightly beyond the level at which learners are well proficient. The gap between “i” and “i +1”is bridged by comprehensible input, which is the information drawn from the context and previous experience.Comprehensible input is considered both necessaiy and sufficient for L2 acquisition to occur and output plays little role.In other words, L2 learning takes place because the data to which learners are exposed contain comprehensible input.

(2) According to Krashen's acquisition-learning hypothesis, there are two ways to approach language learning: acquisition and learning. Acquisition helps us produce natural, rapid, and fluent speech. Learning, which is a conscious study of form, helps us edit this speech. In other words, when we learn something it won't help us produce fluent communication, but it will help us monitor our communication and correct minor errors.

(3) Although the value of input in language learning is self-evident, research on input has been hampered by many problems, one of which is the lack of linguistic analysis of different types of input. It is probably true that various types of input lead to different effects in language learning, but we do not know the actual linguistic differences among these types of input.