The Acquisition- Learning Hypothesis
Krashen makes a careful distinction between Language Acquisition, and Language Learning. According to Krashen, language acquisition is the process of unconsciously gaining knowledge of another language. The reason it is specified that this language be acquired unconsciously is that the learner simply listens to, or reads the language, and gains an underlying mental representation of it, without having to consciously think about rules or structures; with enough exposure, the learner begins to get a feel for what sounds grammatical or ungrammatical, much as a native speaker has intuitions about what sounds right or wrong in their own language.
Learning a language however, is not something that one simply "picks up" rather, it is the process of learning explicit rules, whether through instruction or reading. This learned language is much harder to access in speaking situations, and learners that rely heavily on learned knowledge of a language will be much slower in speaking, for fear of breaking rules. However, as we will see in the next section, learned knowledge of a language still serves an important purpose.
The Monitor Hypothesis
If acquired language is responsible for the raw bulk of language that a speaker may access, it may be helpful to think of the learned portion of a language as the machine that alters and shapes that raw material on its way out. It serves to edit utterances in order to fit the rules better. The use of this machine is most frequent in writing, for although one may encounter many patient listeners when learning a language, patience has a limit, and speakers know that there are unspoken time constraints placed upon conversation. Krashen lists three conditions for use of the machine discussed above which he calls the monitor: 1) sufficient time 2) focus on form and 3) knowledge of the rules. Though the first condition needs little explaining, the second and third are important. One would not be much inclined, much less able, to edit one's own language if a) one didn't care about the language being properly formatted to fit the rules, or b) one simply didn't know the rules. Unlike the behaviorists or cognitive theorists, Krashen doesn't believe that these learned rules will become automatic functions, but rather, these rules may be acquired eventually, allowing for instantaneous use.
The Natural Order Hypothesis
Like many linguists, Krashen believes that the rules and structures of any given language have a natural order in which they are acquired. This order is independent of both ease of rule, and order in which the learner is exposed to the rule. Each person may be slightly different from the next in the order in which they learn a given set of rules, however there are general trends that are noteworthy. One might wonder then, if the order is even close to being predictable, why don't we attempt to teach these rules in the order that they are acquired?
The Input Hypothesis
The answer lies in Krashen's fourth Hypothesis. The input hypothesis stresses that acquisition of a language should occur in situations in which learners are exposed to new structures via natural, comprehensible input, as opposed to exposing learners to a new structure and attempting to drill it into memory. Krashen uses the formula i+1 where 'i' represents the learner's current level and '1' represents the next most basic level of input. The idea is that learners will gradually acquire the new structures provided if they are presented alongside concepts that have already been acquired. Rather than pointing out the structure, the new structure is simply used, in a comprehensible manner, and the learner is expected to gradually acquire it due to the exposure alone.
The Affective Filter Hypothesis
There are however certain conditions which may hinder acquisition. If a learner is exposed to language, via comprehensible input, yet cannot seem to acquire new structures, it may be due to what Krashen calls the 'Affective Filter.' The affective filter is essentially just an abstract way of looking at mental barriers to learning. When there are conditions that prevent acquisition, we consider the filter to be raised, and when conditions are just right, we consider the filter to be lowered. This concept could also be applied to other areas of study, as it's causes are not unique to language learning. A number of factors come into consideration when looking at the affective filter. Some of the things that would cause the filter to be raised might be anxiety, lack of motivation, frustration, or other mental/emotional conditions, whereas learners that feel confident, enjoy speaking the language, and are given the freedom to remain silent, typically have lower filters, and as a result acquire language more readily.
Though many of the hypotheses that Krashen has suggested have gained popularity due to their intuitive nature, they have yet to be shown empirically to be accurate or useful.