- The Acquisition-Learning hypothesis: Krashen claimed that formal instruction, or learning and studying about a language, is a different process from the natural acquisition that takes place as a subconscious act similar to the way children begin to understand their native language. We use learning to produce correct form or grammar, while acquisition is used to understand and produce meaning. As an example, if someone wants to learn Russian, she could learn Russian by taking classes on Russian grammar and vocabulary, but she could acquire the language by reading and listening to Russian texts or living in Russia and interacting with the people and culture in that country (how about interacting with Russian people through the net?).
- The Natural Order hypothesis: There is a natural, predictable order in which people acquire language. It is the same for each person and independent of the instruction program (this is related to Chomky's mentalistic view of language).
- The Monitor hypothesis: The learned system should have the purpose to self-monitor production. It is somehow related to the goal that the learner may be able to identify and correct mistakes or ask for help and reflect on the process of acquisition.
- The Input hypothesis: People acquire a language by receiving comprehensible input. This input should be slightly ahead of a learner’s current state of knowledge (i + 1).
- The Affective Filter hypothesis: When the learner is experiencing high anxiety, low self-esteem or low motivation, the filter turns on and causes the learner to block out input. Learners need both comprehensible input and a weak filter, that is, a learning environment free of stress and anxiety, where the learner is not forced to produce and can progress at his own pace. Krashen suggests that adolescence and puberty may not be good periods for SLA, as this ‘affective filter’ arises out of self-conscious reluctance to reveal oneself and feelings of vulnerability.
"The model has been criticised by many linguists and is no longer considered a valid hypothesis. Its continuing value in the field is only for its historical significance, and the research it has inspired.
McLaughlin (1987) claims that none of the hypotheses is clear in its prediction. For example, the acquisition-learning distinction is not properly defined and the distinction cannot be empirically tested. If only acquired forms can lead to spontaneous speech, as Krashen claims, then it should be impossible for anyone who learns a foreign language in a classroom, and is taught it in their native language, to ever be able to produce spontaneous speech in the target language. This is clearly untrue. Likewise, Krashen provides no criteria for establishing i+1, or for delineating different levels of input..."
Although Krashen doesn't give enough importance in his model to comprehensible output and interaction (he explains why in this article) and we may agree with some part of the criticism, I still think that Krashen's key ideas should be taken into account when deciding on the learning environment and the kind of activities for our students.