* the Monitor hypothesis,
* the Natural Order hypothesis,
* the Input hypothesis,
* the Affective Filter hypothesis.
the Monitor hypothesis
The Monitor hypothesis explains the relationship between acquisition and learning and defines the influence of the latter on the former. The monitoring function is the practical result of the learned grammar. According to Krashen, the acquisition system is the utterance initiator, while the learning system performs the role of the 'monitor' or the 'editor'. The 'monitor' acts in a planning, editing and correcting function when three specific conditions are met: that is, the second language learner has sufficient time at his/her disposal, he/she focuses on form or thinks about correctness, and he/she knows the rule. It appears that the role of conscious learning is somewhat limited in second language performance. According to Krashen, the role of the monitor is - or should be - minor, being used only to correct deviations from 'normal' speech and to give speech a more 'polished' appearance. Krashen also suggests that there is individual variation among language learners with regard to 'monitor' use. He distinguishes those learners that use the 'monitor' all the time (over-users); those learners who have not learned or who prefer not to use their conscious knowledge (under-users); and those learners that use the 'monitor' appropriately (optimal users). An evaluation of the person's psychological profile can help to determine to what group they belong. Usually extroverts are under-users, while introverts and perfectionists are over-users. Lack of self-confidence is frequently related to the over-use of the 'monitor'.
Monitor Over-users. These are people who attempt to Monitor all the time, performers who are constantly checking their output with their conscious knowledge of the second language. As a result, such performers may speak hesitantly, often self-correct in the middle of utterances, and are so concerned with correctness that they cannot speak with any real fluency.
Monitor under-users. These are performers who have not learned, or if they have learned, prefer not to use their conscious knowledge, even when conditions allow it. Under-users are typically uninfluenced by error correction, can self-correct only by using a "feel" for correctness (e.g. "it sounds right"), and rely completely on the acquired system.
The optimal Monitor user. Our pedagogical goal is to produce optimal users, performers who use the Monitor when it is appropriate and when it does not interfere with communication. Many optimal users will not use grammar in ordinary conversation, where it might interfere. (Some very skilled performers, such as some professional linguists and language teachers, might be able to get away with using considerable amounts of conscious knowledge in conversation, e.g. Rivers, 1979, but this is very unusual. We might consider these people "super Monitor users", after Yorio, 1978.) In writing, and in planned speech, however, when there is time, optimal users will typically make
whatever corrections they can to raise the accuracy of their output (see, for example, Krashen and Pon, 1975).
* conversations where the focus is on communication and not form ("Monitor-free").
the Natural Order hypothesis
The Natural Order hypothesis is based on research findings (Dulay & Burt, 1974; Fathman, 1975; Makino, 1980 cited in Krashen, 1987) which suggested that the acquisition of grammatical structures follows a 'natural order' which is predictable. For a given language, some grammatical structures tend to be acquired early while others late. This order seemed to be independent of the learners' age, L1 background, conditions of exposure, and although the agreement between individual acquirers was not always 100% in...