It appears that the role of conscious learning is somewhat limited in second language performance. According to Krashen, the role of the monitor is 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".The Input hypothesis is Krashen's attempt to explain how the learner acquires a second language – how second language acquisition takes place. The Input hypothesis is only concerned with 'acquisition', not 'learning'. According to this hypothesis, the learner improves and progresses along the 'natural order' when he/she receives second language 'input' that is one step beyond his/her current stage of linguistic competence. For example, if a learner is at a stage 'i', then acquisition takes place when he/she is exposed to 'Comprehensible Input' that belongs to level 'i + 1'. Since not all of the learners can be at the same level of linguistic competence at the same time, Krashen suggests that natural communicative input is the key to designing a syllabus, ensuring in this way that each learner will receive some 'i + 1' input that is appropriate for his/her current stage of linguistic competence. The Affective Filter hypothesis embodies Krashen's view that a number of 'affective variables' play a facilitative, but non-causal, role in second language acquisition. These variables include: motivation, self-confidence, anxiety and personality traits. Krashen claims that learners with high motivation, self-confidence, a good self-image, a low level of anxiety and extroversion are better equipped for success in second language acquisition. Low motivation, low self-esteem, anxiety, introversion and inhibition can raise the affective filter and form a 'mental block' that prevents comprehensible input from being used for acquisition. In other words, when the filter is 'up' it impedes language acquisition. On the other hand, positive affect is necessary, but not sufficient on its own, for acquisition to take place.Finally, the less important 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 the studies, there were statistically significant similarities that reinforced the existence of a Natural Order of language acquisition. Krashen however points out that the implication of the natural order hypothesis is not that a language program syllabus should be based on the order found in the studies. In fact, he rejects grammatical sequencing when the goal is language acquisition.
The Role of Grammar in Krashen's View According to Krashen, the study of the structure of the language can have general educational advantages and values that high schools and colleges may want to include in their language programs. Any benefit, however, will greatly depend on the learner being already familiar with the language. It should also be clear that analizing the language, formulating rules, setting irregularities apart, and teaching complex facts about the target language is not language teaching, but rather is "language appreciation" or linguistics, which does not lead to communicative proficiency. he only instance in which the teaching of grammar can result in language acquisition (and proficiency) is when the students are interested in the subject and the target language is used as a medium of instruction. Very often, when this occurs, both teachers and students are convinced that the study of formal grammar is essential for second language acquisition, and the teacher is skillful enough to present explanations in the target language so that the students understand. In other words, the teacher talk meets the requirements for comprehensible input and perhaps, with the students' participation, the classroom becomes an environment suitable for acquisition. Also, the filter is low in regard to the language of explanation, as the students' conscious efforts are usually on the subject matter, on what is being talked about, and not the medium.
