Second language acquisition theory seeks to quantify how and by what processes individuals acquire a second language. The predominant theory of second language acquisition was developed by the University of Southern California's Steven Krashen. Krashen is a specialist in language development and acquisition, and his influential theory is widely accepted in the language learning community.
There are five main components of Krashen's theory. Each of the components relates to a different aspect of the language learning process. The five components are as follows:
This hypothesis actually fuses two fundamental theories of how individuals learn languages. Krashen has concluded that there are two systems of language acquisition that are independent but related: the acquired system and the learned system.
The monitor hypothesis seeks to elucidate how the acquired system is affected by the learned system. When second language learners monitor their speech, they are applying their understanding of learned grammar to edit, plan, and initiate their communication. This action can only occur when speakers have ample time to think about the form and structure of their sentences.
The amount of monitoring occurs on a continuum. Some language learners over-monitor and some use very little of their learned knowledge and are said to under-monitor. Ideally, speakers strike a balance and monitor at a level where they use their knowledge but are not overly inhibited by it.
This hypothesis argues that there is a natural order to the way second language learners acquire their target language. Research suggests that this natural order seems to transcend age, the learner's native language, the target language, and the conditions under which the second language is being learned. The order that the learners follow has four steps:
This hypothesis seeks to explain how second languages are acquired. In its most basic form, the input hypothesis argues that learners progress along the natural order only when they encounter second language input that is one step beyond where they are in the natural order. Therefore, if a learner is at step one from the above list, they will only proceed along the natural order when they encounter input that is at the second step.
This hypothesis describes external factors that can act as a filter that impedes acquisition. These factors include motivation, self-confidence, and anxiety. For example, if a learner has very low motivation, very low self-confidence, and a high level of anxiety, the affective filter comes into place and inhibits the learner from acquiring the new language. Students who are motivated, confident, and relaxed about learning the target language have much more success acquiring a second language than those who are trying to learn with the affective filter in place.
According to second language acquisition theory, the role of grammar in language acquisition is useful only when the learner is interested in learning grammar. Otherwise, Krashen argues that studying grammar equates to language appreciation and does not positively influence language acquisition.