Stephen Krashen is a professor emeritus at the University of Southern California. Krashen’s Monitor Model is a collection of five inter-related hypotheses： the acquisition-learning hypothesis， the monitor hypothesis， the natural order hypothesis， the input hypothesis and the affective-filter hypothesis. These five hypotheses composed the framework of second language acquisition and they greatly influence foreign language teaching.

1. Acquisition-Learning hypothesis

This theory is at the core and is perhaps the most fundamental of Krashen’s theories on SLA. Krashen （1982） had assumed that second language learners have two independent ways in developing linguistics skills of second languages： acquisition and learning. The first is language acquisition， which is similar to the way children develop ability in their first language. Language acquisition is a subconscious process； it focuses on meaningful interaction in the target language and emphasizes the meaning rather than the form. Language acquirers are usually unaware the state that they are acquiring language， but only aware of the fact that they are using the language for communication.

Learning is another way to develop competence in a second language. It refers to that the new language is consciously presented to the learners. They are aware of the grammar and the rules of the new language. It involves error correction and formal instruction （Krashen， 1982）.

However， many linguists regard learning and acquisition as two stages in second language acquisition （Li， 1997）. Learners learn fist and then through lots of practice and memorization to internalize the learning， after which they manage to acquire the language. The acquisition focus on target language environment， but it is not suitable for learners who have no access to the real language context.

2. Monitor Hypothesis

This hypothesis concerns how acquisition and learning are used in second language performance. According to Krashen， the acquisition system is the utterance initiator， while the learning system performs the role of the ‘monitor’ or the ‘editor’. The learning system can help us correct or change the output of the acquired system before we speak or write.

The Monitor Theory only function in some specific conditions： firstly， learners should be given sufficient time to review and follow the rules of their learned system before they output the language. Under most circumstances， students do not have enough time to recall and use rules. Secondly， learners focus on form or thinks about correctness when using the learned knowledge. Even there is enough time， they may focus on what they are saying but ignoring the way saying it. Thirdly， it must be ensured that learners are equipped with an appropriate learned system if they want to use them.

In the process of learning， once “monitor system” works， it plays the role of monitoring and correcting. Language learners will pay special attention to the form of language they are using. Krashen thinks that the purpose of teaching grammar is to cultivate an ideal language monitor （Krashen Terrell， 1983）. He also points out that there is individual variation among language learners when it comes to the use of “monitoring”. Different learners rely on “monitor” to varying degrees.

3. Natural Order Hypothesis

This hypothesis believes that the acquisition of grammatical structures follows a natural but predictable order. When language learners are acquiring a new language， they acquire some grammatical structures or rules earlier and others later. The order has nothing to do with the learner’s age， first language background or exposure to language learning environment， and he rejects grammatical sequence when acquiring a new language.

4. Input Hypothesis

This hypothesis is only discussed under the condition of “acquisition”. It explains the way of acquiring a new language. Krashen suggested that humans acquire language in only one way — by understanding messages or by receiving comprehensible input （Krashen， 1985：2）. The learners improve themselves and make progress when they are naturally picking up some new linguistic input that beyond their present language competence. For example， when learners are exposed to the new language and they hear some meaningful utterance， if they try to understand it， then the acquisition will occur. In this situation， the learners pay more attention to the meaning of utterance but not the form.If we assume the learner is at the stage of “i”， then the acquisition occurs when he or she is exposed to “Comprehensible Input”. The “Comprehensible Input” belongs to the level that beyond the learners’ current linguistic ability， and we call it “i+1”. Considering that different learners are at different levels， Krashen suggests that natural communicative input is the key to design a syllabus， which should be gradable. In this way， different learners are possible to achieve their “i+1” input matching their current stage of linguistic competence. However， the Input Hypothesis cannot be fully utilized in teaching. It is difficult to define the “i” in “i+1”. There is no general standard to measure each student’s level （Xu， 2011）.

5. Affective-Filter Hypothesis

The Affective Filter Hypothesis means that various affective factors， such as motivation， self-confidence and anxiety， play a facilitative but non-causal role in second language acquisition. Krashen （1982） claims that lacking motivation， low self-confidence， anxiety， and so on， can combine to “raise the filter” to form a “mental block”， which prevents Comprehensible Input from reaching the language acquisition device （LAD）， then the acquisition fails. Comprehensible input is only a necessary but not sufficient condition to achieve acquisition. Language learner should not be over upset about the failure in language acquisition or highly esteem themselves in acquiring the target language.

Krashen （1985） stated： “We can summarize the five hypotheses with a single claim： people acquire a second language only if they obtain comprehensible input and if their affective filters are low enough to allow the input ‘in’. When the filter is ‘down’ and appropriate comprehensible input is presented （and comprehended）， acquisition is inevitable”.

References

[1]Krashen， S. （1982）， Principles and Practice in Second Language Acquisition. Oxford： Pergamon Press.

[2]Krashen， S. Terrell （1983）， The Natural Approach： Language Acquisition in the Classroom. Oxford： Pergamon Press.

[3]Krashen S. The Input hypothesis： Issues and Implications. London： Longman，1985.

[4]許珊. （2011）. 近二十年國內(nèi)學(xué)者對克拉申輸入假說研究綜述. 邊疆經(jīng)濟(jì)與文化（5）， 136-137.