某天查找 “外語習得” 資料時，看到網站上這樣一段介紹：
“Stephen D Krashen 是美國南加州大學教育學院榮譽退休教授，最知名的研究是建立第二語言學習的普遍性理論，也是自然研究法的倡立者之一。1977 年他在洛杉磯威尼斯海灘獲得俯臥撐大賽的冠軍，還持有跆拳道黑帶。他的著作《閱讀的力量》雖然只有 163 頁，卻有 366 種參考文獻，並被讀者認爲應該是教育局官員和中小學老師人手一本的必備參考書。”
被逗樂了。尤其是想像語言學家在沙灘上跟大家比賽俯臥撐的樣子。
馬上找來Krashen教授的照片，原來是這樣一位正經的小老頭。
以下譯自www.sk.com.br文章《Stephen Krashen's Theory of Second Language Acquisition》
語言習得並不需要大量使用刻意的語法規則，也不需要進行枯燥的練習。
它只要求能用該語言進行有意義的交互——自然狀態的交流——說話人不會關心自己的語言形態，而是專注於傳遞和理解的信息本身。
……“理解性輸入” 是語言習得的關鍵和必要點。
因此，最好的方法就是在低焦慮狀態下所提供的 “理解性輸入”，其中包含學生真正期待的信息量。這種方法不會強迫學生用外語在早期遣詞造句，而允許他們到 “成熟” 時再這麼做，因爲認識到只有具備了交流性和理解性的輸入，才能帶來語言學習的進步，而不是靠強迫和改正。
在現實中，如果有具備同理心的母語者願意來進行對話練習，將對語言學習者大有幫助。
導論
Stephen Krashen (南加州大學) 是語言領域的專家，專注於語言習得與進步理論。他的很多新近理論都影響了非英語和雙語習得研究。在過去20年間，他出版了100多部書籍論文，被邀請在美國和加拿大進行了300多場講座。
這是一篇關於Krashen的二語習得理論的簡述。他的研究廣爲人知、頗受好評，帶給1980年代以來的外語研究和教學很大影響。
Krashen二語習得理論的論述
Krashen的二語習得理論包含5個重要假設：
習得-學習假設 （the Acquisition-Learning hypothesis）;
監督假設（the Monitor hypothesis）;
自然序列假設（the Natural Order hypothesis）;
輸入假設（the Input hypothesis）;
情感過濾假設（the Affective Filter hypothesis）.
習得-學習 的區別是Krashen理論中最基本的假設，在語言學家和語言練習者中也流傳最廣。在他看來，外語表現有兩個獨立系統：“習得系統”和“學成系統”。“習得系統”（ 或 “習得”）是下意識過程的產物，和兒童學習母語的經歷非常像。它要求能用該語言進行有意義的交互——自然狀態的交流——說話人不會關心自己的語言形態，而是專注於交流行爲。
“學成系統”（或 “學習”）是形式教學的產物，包含一個有意識的過程，來自關於語言的刻意知識，比如語法規則。在Krashen看來，“習得” 比 “學習” 更重要。
監督假設，解釋了習得和學成二者之間的關係，並且定義了後者對前者的影響。監督功能是已學語法的實際結果。Krashen認爲，習得系統是話語發射器，而學成系統則扮演 “監視器” 或 “編輯器” 的角色。“監視器” 在計劃、編輯和糾正功能中起作用。
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觀點中語法的角色
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.
參考書籍：
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.