某天查找 “外语习得” 资料时，看到网站上这样一段介绍：
“Stephen D Krashen 是美国南加州大学教育学院荣誉退休教授，最知名的研究是建立第二语言学习的普遍性理论，也是自然研究法的倡立者之一。1977 年他在洛杉矶威尼斯海滩获得俯卧撑大赛的冠军，还持有跆拳道黑带。他的著作《阅读的力量》虽然只有 163 页，却有 366 种参考文献，并被读者认为应该是教育局官员和中小学老师人手一本的必备参考书。”
被逗乐了。尤其是想像语言学家在沙滩上跟大家比赛俯卧撑的样子。
马上找来Krashen教授的照片，原来是这样一位正经的小老头。
以下译自www.sk.com.br文章《Stephen Krashen's Theory of Second Language Acquisition》
语言习得并不需要大量使用刻意的语法规则，也不需要进行枯燥的练习。
它只要求能用该语言进行有意义的交互——自然状态的交流——说话人不会关心自己的语言形态，而是专注于传递和理解的信息本身。
……“理解性输入” 是语言习得的关键和必要点。
因此，最好的方法就是在低焦虑状态下所提供的 “理解性输入”，其中包含学生真正期待的信息量。这种方法不会强迫学生用外语在早期遣词造句，而允许他们到 “成熟” 时再这么做，因为认识到只有具备了交流性和理解性的输入，才能带来语言学习的进步，而不是靠强迫和改正。
在现实中，如果有具备同理心的母语者愿意来进行对话练习，将对语言学习者大有帮助。
导论
Stephen Krashen (南加州大学) 是语言领域的专家，专注于语言习得与进步理论。他的很多新近理论都影响了非英语和双语习得研究。在过去20年间，他出版了100多部书籍论文，被邀请在美国和加拿大进行了300多场讲座。
这是一篇关于Krashen的二语习得理论的简述。他的研究广为人知、颇受好评，带给1980年代以来的外语研究和教学很大影响。
Krashen二语习得理论的论述
Krashen的二语习得理论包含5个重要假设：
习得-学习假设 （the Acquisition-Learning hypothesis）;
监督假设（the Monitor hypothesis）;
自然序列假设（the Natural Order hypothesis）;
输入假设（the Input hypothesis）;
情感过滤假设（the Affective Filter hypothesis）.
习得-学习 的区别是Krashen理论中最基本的假设，在语言学家和语言练习者中也流传最广。在他看来，外语表现有两个独立系统：“习得系统”和“学成系统”。“习得系统”（ 或 “习得”）是下意识过程的产物，和儿童学习母语的经历非常像。它要求能用该语言进行有意义的交互——自然状态的交流——说话人不会关心自己的语言形态，而是专注于交流行为。
“学成系统”（或 “学习”）是形式教学的产物，包含一个有意识的过程，来自关于语言的刻意知识，比如语法规则。在Krashen看来，“习得” 比 “学习” 更重要。
监督假设，解释了习得和学成二者之间的关系，并且定义了后者对前者的影响。监督功能是已学语法的实际结果。Krashen认为，习得系统是话语发射器，而学成系统则扮演 “监视器” 或 “编辑器” 的角色。“监视器” 在计划、编辑和纠正功能中起作用。
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".
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 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 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.
Finally, the fifth hypothesis, 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 and anxiety. Krashen claims that learners with high motivation, self-confidence, a good self-image, and a low level of anxiety are better equipped for success in second language acquisition. Low motivation, low self-esteem, and debilitating anxiety can combine to '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.
Krashen观点中语法的角色
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. It should be clear, however, that examining irregularity, formulating rules and teaching complex facts about the target language is not language teaching, but rather is "language appreciation" or linguistics.
The 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.
This is a subtle point. In effect, both teachers and students are deceiving themselves. They believe that it is the subject matter itself, the study of grammar, that is responsible for the students" progress, but in reality their progress is coming from the medium and not the message. Any subject matter that held their interest would do just as well.
参考书籍：
Crystal, David The Cambridge Encyclopedia of Language. Cambridge University Press, 1997.
Krashen, Stephen D. Principles and Practice in Second Language Acquisition.Prentice-Hall International, 1987.
Krashen, Stephen D. Second Language Acquisition and Second Language Learning. Prentice-Hall International, 1988.