Input Hypothesis - Acquisition-learning Hypothesis

In modern linguistics, there are many theories as to how humans are able to develop language ability. According to Stephen Krashen's acquisition-learning hypothesis, there are two independent ways in which we develop our linguistic skills: acquisition and learning. This theory is at the core of modern language acquisition theory, and is perhaps the most fundamental of Krashen's theories.

Acquisition of language is a subconscious process of which the individual is not aware. One is unaware of the process as it is happening and when the new knowledge is acquired, the acquirer generally does not realize that he or she possesses any new knowledge. According to Krashen, both adults and children can subconsciously acquire language, and either written or oral language can be acquired. This process is similar to the process that children undergo when learning their native language. Acquisition requires meaningful interaction in the target language, during which the acquirer is focused on meaning rather than form.

Learning a language, on the other hand, is a conscious process, much like what one experiences in school. New knowledge or language forms are represented consciously in the learner's mind, frequently in the form of language "rules" and "grammar" and the process often involves error correction. Language learning involves formal instruction, and according to Krashen, is less effective than acquisition.