On Krashen’s Second Language Acquisition TheoriesAbstract : Krashen’s Second Language Acquisition Theories consist of five main hypothesizes : the acquisition-learning hypothesis ,the natural order hypothesis ,the monitor hypothesis ,the input hypothesis ,and the affective fliter hypothesis .These hypothesizes have certain directive significance to teaching practice ,but also has some limitations : ignore the output acquisition process ,lack of operability in actual teaching in some degree.Key words : acquisition-learning hypothesis , natural order hypothesis , monitor hypothesis ,input hypothesis ,and affective fliter hypothesisⅠ.IntroductionAs a part of Second Language Acquisition ,Krashen’s Second Language Acquisition Theories is very significant in Second Language Acquisition According to his theories second acquisition has two quite different systems : acquisition and learning systems .Acquisition is the subconscious development of language rules ,just like in first language development .It focuses on meaning ,instead of on language form . Learning involves formal knowledge (and teaching ) of a language ,including error correction .It is a conscious development of languagerules .Krashen argues that error correction and learning does not lead to acquisition of fluency ,and that acquisition is more important than learning .Next we will have detailed analysis of his hypothesizes .Ⅱ.General Understanding of Krashen’ Second Language Acquisition TheoriesAcquisition-Learning HypothesisThis hypothesis is what Krashen considers to be perhaps the most fundamental of all hypothesizes .And it has been highly influential, and still remains the source of much debate today. The starting point ,also the core of this hypothesis is the distinction between acquisition and learning, and the different roles they play in second language acquisition. When learners acquire a second language they will internalize the language rulesubconsciously .They will not pay much attention to the language forms ,but to the meaning they want to express and understand. Learning (usually refers to the formal classroom context ) is the process of conscious study of language uncover knowledge ,such as language rules. It is systematical and formal. According to Krashen, only acquired knowledge is readily available for natural fluent communication, and learned knowledge can only play as a monitor using language.Monitor HypothesisThis hypothesis has a close relationship with acquisition –learning hypothesis. It states that ,only language acquisition system ,also the potential language knowledge ,is the real knowledge competence, and that conscious learning has extremely limited function in adult L2 performance. Acquisition” innates “ the speaker’s utterances and is responsible for fluency .While learning has only one function –as a monitor or editor, making minor changes and polishing what the acquired system hasproduced .This monitor comes before ,while ,and after the language output (speaking and writing).There are three kinds of monitor users ,monitor over-users ,monitor under users ,and monitor optimal users. Learners should properly use this monitor according to their personal situations. While have a conversation with others, we should focus on meaning not language forms. And the conversation will not be fluent if we overuse the monitor. However, while writing it is necessary for us to use the monitor to avoid expressing faults. There are three conditions for us to use monitor：Time (best applied when there is enough time)Focus on form (not on communication)Know the rules.Natural Order HypothesisStudents acquire (not learn) grammatical structures in a predictable order, that is certain grammatical structures tend to be learned early and others later. It appears that the order of acquisition for first language acquisition is not identical for second language acquisition, but there are some similarities. The naturalorder hypothesis reflects Noma Chomsky’s revolutionary notion that we all have a built-in Language Acquisition Device (LAD).Because of the LAD we tend to learn different language structure at different levels/stages as young learners ( also the same for older learners).Equipped with this innatedevice ,language learning follows a its natural route .For example, when children and adults learn English as a second language, they will acquire the present continue tense earlier than past tense, and 名词复数 earlier than 名词所有格Input HypothesisAccording to Krashen , the input hypothesis attempts to answer what is perhaps the most important question in Second Language Acquisition filed ,that is ,how we acquire language .Thus this notion has become the central part of Krashen’s theories. The input hypothesis states that one can acquire language in only one way – by exposure to comprehensible input. If the input contains forms and structures just beyond the learners’ current level of competence in the language (what Krashen calls i+1).Then both comprehension and acquisition will occur. Input which is either too simple (i + i) or too complex (i + 2/3/4) will not be useful for acquisition.” i” represents learners’ current language level .1 represents the input that is slightly higher than learners’ current level. Krashen also provide the requirements of the idealinput :first,the input should be comprehensible. Second, it should be sufficient .third, it should be interesting and relevant to learners.Affective FilterKrashen believes that comprehensible alone is not sufficient for language acquisition. Learners also need to” let that input in “ .The affective filter determines how receptive to comprehensive input a learner is going to be. Learners with high motivation ,high self-confidence ,a good self- image ,and a low level of anxiety are better equipped for success in Second Language .Low motivation, ,low self- esteem ,and debilitating anxiety can combine to raise “affective filter” and form a “mental block” that prevents comprehensive input from entering the individual’s language acquisition device. The hypothesis is based on the theory of an “affective filter “ which states that successful secondlanguage acquisition depends on the learners’ feeling. Negative attitudes are said to act as a filter ,preventing the learners from making use of input, and thus hindering success in language learning. So to help students let that input in ,teachers should try to creat positive atmosphere in language teaching, try to activate learners’ positive feeling ,like interest ,good motivation, and attitude and desire to learn by encouragement, humanitycare ,vivid design of teaching ,extra-curricular activities.Ⅲ.ConclusionKrashen’s theories are very significant in that ,first , it makes a clear distinction between acquisition and learning ,second , it emphases the importance of the comprehensibility of the input , and that it should be slightly higher than learners’ present level, third , it attaches importance to learners’ feeling factors, like motivation, interest , anxiety and ect , and their influence on learners’ acquisition of a second language.。