Language acquisition occurs via two main mechanisms

Language acquisition occurs via two main mechanisms; the native language, first language acquisition (FLA) or a foreign language in addition to L1. In this case it is called second/foreign language acquisition (SLA). Both processes are achieved through different means, but the main focus in this paper is on the latter; SLA. Krashen (1981) thinks that speaking the foreign language promotes acquisition, and conversation in which the acquirer has some sort of control over the topic and in which the other participants exert an effort to make themselves understood provide valuable intake.
Krashen believes that the best activities for the classroom are those that are natural, interesting and comprehensible. He claims that if the teaching programme can provide these characteristics then the classroom may be the best place for L2 acquisition, up to the intermediate level. Similarly, Littlewood (1984, 59) considers “the ideal input for acquiring a second language is similar to the input received by the child, comprehensible, relevant to their immediate interests, not too complex, but not strictly graded either”. Krashen (1982) presents a set of requirements that should be met by any activity aiming at subconscious language acquisition. In this respect, he considers comprehensible input the most important factor for language acquisition, and he regards incomprehensible input as a factor that hinders L2 acquisition. According to him, this explains why educational T.V. programmes fail to teach foreign languages unless the acquirer speaks “a very closely related language”. These factors have led Krashen to define the good language teacher as “someone who can make input comprehensible to a non-native speaker, regardless of his or her level of competence in the target language” (ibid,64).
Krashen (1982) also strongly believes that the best input is so interesting, natural and relevant that the acquirer (the learner) may even “forget” that the message is encoded in a foreign language. In addition, optimal input is not grammatically structured. When we focus on grammatical considerations, there will be less genuinely interesting input. He maintains that optimal input must be in sufficient quantity. It seems clear to him, however, that much time should be devoted to supplying comprehensible input, and that would stimulate more rapid second language acquisition in that the acquirer can get more of the target language (Hasan, 208).

2. Language Acquisition
Bylanguage acquisition, we mean the process of learning and development of a person’s language (Fletcher, 1985). It is the process by which humans acquire the capacity to perceive and comprehend language, as well as to produce and use words and sentences in order to communicate. The learning of a native or first language (L1) is called first language acquisition(FLA), while the term second language acquisition (SLA) is used to refer to the learning of a second or foreign language (L2) (Mansoor, 2014:137-138).
It is important to make a distinction between acquisition and learning. Byacquisition,we meanthe gradual development of ability in a language by using it naturally and subconsciously in communicative situations with those who know and speak the language. Learning, on the other hand, refers to a more conscious process ofstudying a language, usually in an institutional setting (through instruction). Thus, the first term refers to L1, while the second refers to L2 (ibid).
2.1 Second Language Acquisition
SLAis a term used to refer to the processes by which people develop proficiency in a second or foreign language. Historically, SLA began as an interdisciplinary field(Gass and Selinker, 2008: 1). However, the early beginning of this field goes backto Corder’s(1967) essay The Significance of Learners’ Errors, and Selinker’s (1972) article Interlanguage(VanPatten & Benati, 2010: 2-5).In the 1970s the general trend in SLA was for research exploring the ideas of Corder and Selinker, and refuting behavioristtheories of language acquisition.
By the 1980s, the theories of Stephen Krashen had become the prominent paradigm in SLA. In his theory, known as the Input Hypothesis, Krashen suggested that language acquisition is basically achieved by what he called comprehensible input; language input that learners can understand. Krashen’s model was influential in the field of SLA and also had a large influence on language teaching, but it left some important processes in SLA unexplained. Research in the 1980s was characterized by the attempt to fill in these gaps. Some approaches included White’s descriptions of learner competence, and Pienemann’s use of speech processing models and lexical functional grammar to explain learner output (ibid).
The 1990s witnessed the introduction of sociocultural theory, an approach to explain second-language acquisition in terms of the social environment of the learner.In the 2000s research was split into two main camps of linguistic and psychological approaches. VanPatten and Benati do not see this state of affairs as changing in the near future, pointing to the support both areas of research have in the wider fields of linguistics and psychology, respectively (ibid, cited in wikipedia, 2018).

3.Krashen’s Theory of SLA
Krashen’s theory of second language acquisition consists of five main hypotheses (Krashen, 1982: 10-32). These include:

3.1 The Acquisition-Learning Hypothesis
The Acquisition-Learning distinction is the most fundamental of all the hypotheses in Krashen’s theory and the most widely known among linguists and language practitioners. According to Krashen there are two independent systems of second language performance: ‘the acquired system’ and ‘the learned system’. The ‘acquired system’ or ‘acquisition’ is the product of a subconscious process very similar to the process children undergo when they acquire their L1. It requires meaningful interaction in the target language, natural communication, in which speakers concentrate not on the form of their utterances, but on the communicative act.
The “learned system” or “learning” is the product of formal instruction and it comprises a conscious process which results in conscious knowledge about the language, for example knowledge of grammar rules. According to Krashen learning is less important than acquisition (ibid).

3.2 The Monitor Hypothesis
The Monitor Hypothesis explains the relationship between acquisition and learning and defines the influence of the latter on the former. The monitoring function is the practical result of the learned grammar. 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: 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) (ibid).

3.3 The Natural Order Hypothesis
The Natural Order Hypothesis is based on research findings (Dulay & Burt, 1974; Fathman, 1975; Makino, 1980 cited in Krashen, 1982) which suggest 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 and conditions of exposure. Although the agreement between individual acquirers was not alwaysdefinitivein 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.

3.4 The Input Hypothesis
The InputHypothesis is Krashen’s attempt to explain how the learner acquires a second language andhow second language acquisition takes place. He puts it in this way:”The important question is: How do we acquire language?”(Krashen, 1982: 20).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'(where i represents current competence), then acquisition takes place when he/she is exposed to ‘Comprehensible Input’ that belongs to level ‘i + 1’ (where I represents the next level)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 (ibid).

3.5 The Affective Filter Hypothesis
According to Krashen there are a number of affective variablesthat play a facilitative, but non-causal role in second language acquisition. These variables include: motivation, self-confidence and anxiety (Krashen, 1981). 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 greatanxiety 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(www.sk.com).

4. The Role of Input
In normal conditions, infants are helped in their language acquisition by the typical behaviour of older children and adults in the home environment who provide language samples or input for the child. This input is usually a simplified speech style given by someone who spends a lot of time interacting with the child (like parents or caregivers). This speech which is addressed to young children by the adults or older children who are looking after them is called motherese or caregiver speech(Snow & Ferguson, 1977; Richards, et al, 1993:45; Yule, 2010:171).
This kind of speech is also referred to as comprehensible inputthat can be understood by listeners although they do not understand all the words and structures in it. According to Krashen(1982, 62), giving learners this kind of input helps them acquire language naturally, rather than learn it consciously.

4.1Characteristics of Optimal Input
Krashen (1982) defined that optimal input should be comprehensible, be interesting and/or relevant, not be grammatically sequenced and be in sufficient quantity. If the leaner can be exposed to input having these characteristic features, it is considered that acquisition is more likely to occur at its best possible levels.

4.1.1Comprehensible
Krashen (1982, 63) consider this as the most important input characteristic. To him, when the acquirer does not understand the message, there will be no acquisition. This means that incomprehensible input is merely noise which doesn’t help in the process of SLA. According to information processing theory concerning comprehension and production, if the learner cannot keep up with the rate of exposure and the input content is far beyond his linguistic competence, he/she will fail to comprehend and therefore, to acquire(Carroll,1990, cited in Wang, 2010). Therefore, the teacher must ensure that the material he/she chooses is not so demanding on the student.A “defining characteristic of a good teacher is someone who can make input comprehensible to a non-native speaker, regardless of his or her level of competence in the target language” (Krashen, 1982: 64). Krashen maintains that the main function of all teachers is to try to help make input comprehensible (both linguistically and non-linguistically); to do for the adult learner what the outside world cannot or will not do (ibid).
Linguistically,studies have shown that many things can be done to make the speech more comprehensible to less competent speakers. Hatch, 1979 (cited in Krashen, 1982: 64) has summarized the linguistic aspects of simplified input which appear to promote comprehension as follows:
1.Slower rate and clearer articulation, which helps acquirers to identify word
boundaries more easily, and allows more processing time.
2. More use of high frequency vocabulary, less slang, fewer idioms.
3. Syntactic simplification, shorter sentences.
Non-linguistically, providing extra-linguistic support in the form of realia (real objects) and pictures for beginning classes have proved to be helpful devices to encourage SLA. Good teachers also take advantage of the student’s knowledge of the world in helping comprehension by discussing topics that are familiar to the student. Discussing or reading about a topic that is totally unknown to the learner will make the message harder to understand, and consequently hinders acquisition.