Krashen's theory of second language acquisition consists of five main hypotheses:

• the Acquisition-Learning hypothesis;
• the Monitor hypothesis;
• the Input hypothesis;
• and the Affective Filter hypothesis;
• the Natural Order hypothesis.

The Acquisition-Learning distinction is the most fundamental of the five hypotheses in Krashen's theory and the most widely known among linguists and language teachers. According to Krashen there are two independent systems of foreign 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 first language. It requires meaningful interaction in the target language - natural communication - in which speakers are concentrated not in the form of their utterances, but in 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. A deductive approach in a teacher-centered setting produces "learning", while an inductive approach in a student-centered setting leads to "acquisition".

According to Krashen 'learning' is less important than 'acquisition'. (See here our in-depth analysis of the Acquisition/Learning hypothesis and its implications).

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 their disposal.
• They focus on form or think about correctness.
• They know 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 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). An evaluation of the person's psychological profile can help to determine to what group they belong. Usually extroverts are under-users, while introverts and perfectionists are over-users. Lack of self-confidence is frequently related to the over-use of the "monitor".

The Input hypothesis is Krashen's attempt to explain how the learner acquires a second language – how second language acquisition takes place. 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', then acquisition takes place when he/she is exposed to 'Comprehensible Input' that belongs to level 'i + 1'. 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.

See here an enlightening video by Krashen about comprehensible input.