Second language acquisition (SLA), also referred to as second language learning, is the process of developing the ability to use a language other than that native to the learner. There are several theoretical approaches to second language learning, which are concerned with the processes and ways in which a learner acquires the skills and knowledge needed to use the language aptly.
One theory of SLA prominent in the field of linguistics from the 1940s to the 1960s is Behaviourism. This approach focuses primarily the formation of habits by the learner through both mental and environmental processes. The external environment can pose a stimulus and input for learning, enabling the learner to make associations between objects they see, words they hear and meaning. There are also several mental processes of habit formation; firstly, the learner receives comprehensible aural and audio-lingual input, which is then processed mentally by means of repetition, memorisation and practice until the learner is capable of producing comprehensible output. These processes should start with simple concepts and progress onto more complex ones as the learner develops good habits, similarly to the structure of first language acquisition. In terms of second language learning, a learner's native language (L1) is thought to act as base for the second language being learnt (L2). Roberts Lado (1957) developed the hypothesis of Contrastive Analysis (CA), stemming from the behaviourist approach. Through contrastive analysis linguists can identify similarities and differences between L1 and L2 both grammatically and structurally. Similarities between the two languages can lead to positive transfer, where certain structures and parts of the language are similar to that of L1, therefore the learner finds it easier to acquire the new ideas of L2. Negative transfer on the other hand occurs when difference between L1 and L2 cause difficulty for the learner, as the habits formed in L1 need to be disregarded in order to acquire certain concepts of L2.By identifying these differences it is thus possible to predict certain difficulties that learners of a second language could be expected to face, and therefore possible to adapt and focus the teaching methods and materials of that language in order to help the learner avoid the formation of bad habits. This hypothesis also led to the theory of Error Analysis (EA) One cause of errors made is the negative transfer from L1, which causes interference with L2 learning. However this is not always a negative aspect of learning, as Stephen Cordner (1963) proclaimed- errors are indispensible to L2 learners. Errors can be seen as useful indicators of the level of competence of the learner, highlighting areas of difficulty and areas that remain to be learnt, key indicators of second language learning. Cordner (1963) also made a distinction between errors and mistakes. An error can be considered a gap in knowledge as a result of something that a learner is yet to learn. A mistake could be considered an error made due to the learner forgetting or misusing something that has already been learnt. With regards to EA reinforcement and feedback is another important part of the behaviourist approach to second language learning. This is the idea that output should be either encouraged with praise and reward or discouraged depending on the nature of it.
Another theory of second language learning is Mentalist, also known as the Natural Approach. This theory is based on the idea that there is a natural order in second language learning, similar to that found in the first language acquisition of a child. Extensive research was carried out in the 1960s regarding the theory of first language acquisition. Chomsky (1959) challenged the aforementioned theory of behaviourism in stating that children did not only acquire language through imitation, but instead through an innate knowledge of grammar, based on the idea that everybody has genetically determined language acquisition device, a programmed part of the brain specified for language learning. This theory is known as the theory of Universal Grammar (UG), whereby a child not only has inner knowledge of how the grammar related to L1 works, but also a knowledge which can be applied to any second language learnt. Chomsky believes that the a child has the ability to compare the input they receive against their pre-programmed knowledge in the brain, thus producing output which is not only the product of repetition and imitation. This theory also incorporates the idea that there is a natural order of acquisition, which every child goes through in both FLA and SLA. This is a useful theory, as knowledge of the sequences in which a child learns its first language can then be applied to the teaching of second languages to help the learner develop and move closer towards the level of a native speaker in their own natural time. Another linguist in this field who supported this language was Stephen Krashen. He formulated the Monitor Model, and a set of 5 hypothesises which have helped to formulate methodologies of language teaching. One of which is the aforementioned idea of natural order. He also made a distinction between acquisition and learning. Acquisition is said to form the basis of fluency through communication and subconscious processing of language, and learning is the conscious processing of set forms and rules. Another of his hypothesises is the learner's need for comprehensible input in order to develop through language learning. He states that this input should be just above the level of the learner's competence. I.e. if the learners competence level is 'I', they should be exposed to teaching methods and materials that are equal to 'I' + 1 in order for them to make good progress, following the theory of natural order of acquisition. The monitor model is based on the ideas presented by Chomsky where by everyone has the ability to acquire an inner mental device known as a monitor. This monitoring device enables a learner to correct mistakes in their own speech and writing and Krashen identified 3 kinds of monitor users. Over-users are overly concerned with accuracy, thus not particularly fluent. Under-users tend to not monitor their output at all so fluency takes priority over accuracy. Optimal users monitor output with the focus is on fluency and accuracy. Krashen's final hypothesis identified an imaginary barrier that could hinder the learning of a person, something that is formally known as the affective filter. This is the theory that affective factors can affect how learners process input, thus affecting their own output. Factors that are known to raise the affective filter of a learner, thus hindering their learning, include feelings and emotions such as anger, anxiety, boredom, self-consciousness, stress and lack of motivation. Therefore with regard to the implications for language teaching methodologies, it is apparent that the teacher should strive to minimise the affective factors in creating a language learning environment that is relaxing, and use teaching methods which keep the students engaged and motivated.
A third theoretical approach to language learning is known as the Interactionist approach. This approach focuses on the way language is developed through communication, interaction and conversation. Further to Krashen's input hypothesis as part of the structuralist approach to language learning, Long (1983) developed the interaction hypothesis. His beliefs are that a learner developed their knowledge of learning by filling in gaps in their knowledge through the communication and interaction with other people speaking the language, for example native speakers. During a conversation between two people, a gap in the knowledge of one leads to a pause in flow; this pause in flow of conversation then leads to the opportunity for learning known as the negation of meaning. Where two people negotiate what meaning and by reaching a conclusion fills in the gap in the learner's knowledge, thus aiding the development of language, as the more gaps that are filled, the more fluent a person will become. With regards to this approach to language learning, the type of input is an important factor. Input can be categorised by type and aims. When the input focuses primarily on language it is known as meta-linguistic, other types of input are used to focus primarily on meaning with less focus on grammatical accuracy. Whichever type of input is used, it is important for the learner's development that the input is comprehensible. Long (1983) formulated a set of devices which are intended to ensure comprehensible input for the learner. The first set of devices concerns the way in which the teacher approaches topics. He stated that these should be interesting in order to gain the learner's interest and motivation, they should not be taken into too much depth and understanding should be ensured. Other ways to reduce the probability of trouble arising is for the teacher to use a slow pace when talking and explaining topics, to put emphasis on key words and terms and also to emphasise and repeat utterances by both other learners and themselves. All of these things, according to Long, are likely to help the learner to optimally develop their language learning. If the learners do face problems with the input, Long's theory was that the teacher should give control to the student which regards to the topic, for example if there is an unintentional change of topic the teacher should let the student carry on with it, as opposed to stopping them and bringing them straight back to topic. This is a form of positive reinforcement. Furthermore if a teacher is unsure about a student's meaning then they should request further explanation and allow for ambiguity. This builds the learners confidence and reduces negative feedback.
It is also important to note that the output from the learners varies amongst the aforementioned approaches to language learning. Behaviourist approaches hold greater importance on the grammatical accuracy of output, whereas mentalist and interactionist approaches focus more on the meaning conveyed as opposed to complete accuracy. Swain outlined 3 functions of output. One of his arguments, particularly important to the interactionist approach, is that output within a conversational environment allows the learner to identify gaps in their own knowledge and therefore personal development through experience as the learner can then improve on what they don't know. This interaction can be between both two or more people, or just a learner and the environment itself.