There are five key hypotheses about second language acquisition by Krashen:
1. THE ACQUISITION-LEARNING DISCTINCTION
Adults have two different ways to develop competence in a language: language acquisition and language learning.
Language acquisition is a subconscious process not unlike the way a child learns language. Language acquirers are not consciously aware of the grammatical rules of the language, but rather develop a "feel" for correctness. "In non-technical language, acquisition is 'picking-up' a language."
Language learning, on the other hand, refers to the "conscious knowledge of a second language, knowing the rules, being aware of them, and being able to talk about them." Thus language learning can be compared to learning about a language.
The acquisition-learning distinction hypothesis claims that adults do not lose the ability to acquire languages the way that children do. Just as research shows that error correction has little effect on children learning a first language, so too error correction has little affect on language acquisition.
2. THE NATURAL ORDER HYPOTHESIS
The natural order hypothesis states that "the acquisition of grammatical structures proceeds in a predictable order." For a given language, some grammatical structures tend to be acquired early, others late, regardless of the first language of a speaker. However, as will be discussed later on in the book, this does not mean that grammar should be taught in this natural order of acquisition.
3. THE MONITOR HYPOTHESIS
The language that one has subconsciously acquired "initiates our utterances in a second language and is responsible for our fluency," whereas the language that we have consciously learned acts as an editor in situations where the learner has enough time to edit, is focused on form, and knows the rule, such as on a grammar test in a language classroom or when carefully writing a composition. This conscious editor is called the Monitor.
Different individuals use their monitors in different ways, with different degrees of success. Monitor Over-users try to always use their Monitor, and end up "so concerned with correctness that they cannot speak with any real fluency." Monitor Under-users either have not consciously learned or choose to not use their conscious knowledge of the language. Although error correction by others has little influence on them, they can often correct themselves based on a "feel" for correctness.
Teachers should aim to produce Optimal Monitor users, who "use the Monitor when it is appropriate and when it does not interfere with communication." They do not use their conscious knowledge of grammar in normal conversation, but will use it in writing and planned speech. "Optimal Monitor users can therefore use their learned competence as a supplement to their acquired competence."
4. THE INPUT HYPOTHESIS
The input hypothesis answers the question of how a language acquirer develops competency over time. It states that a language acquirer who is at "level i" must receive comprehensible input that is at "level i+1." "We acquire, in other words, only when we understand language that contains structure that is 'a little beyond' where we are now." This understanding is possible due to using the context of the language we are hearing or reading and our knowledge of the world.
However, instead of aiming to receive input that is exactly at our i+1 level, or instead of having a teacher aim to teach us grammatical structure that is at our i+1 level, we should instead just focus on communication that is understandable. If we do this, and if we get enough of that kind of input, then we will in effect be receiving and thus acquiring out i+1. "Production ability emerges. It is not taught directly."
Evidences for the input hypothesis can be found in the effectiveness of caretaker speech from an adult to a child, of teacher-talk from a teacher to a language student, and of foreigner-talk from a sympathetic conversation partner to a language learner/acquirer. One result of this hypothesis is that language students should be given a initial "silent period" where they are building up acquired competence in a language before they begin to produce it.
Whenever language acquirers try to produce language beyond what they have acquired, they tend to use the rules they have already acquired from their first language, thus allowing them to communicate but not really progress in the second language.
5. THE AFFECTIVE FILTER HYPOTHESIS
Krashen views that a number of 'affective variables' play a facilitative, but non-causal, role in second language acquisition. These variables include: motivation, self-confidence and anxiety. 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 debilitating anxiety 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.