In the field of Second Language Acquisition, there are many theories about the most effective way for language learners to acquire new language forms. One theory of language acquisition is the Comprehensible Output Hypothesis.
Developed by Merrill Swain, the comprehensible output hypothesis states that learning takes place when a learner encounters a gap in his or her linguistic knowledge of the second language (L2). By noticing this gap, the learner becomes aware of it and may be able to modify his output so that he learns something new about the language. Although Swain does not claim that comprehensible output is solely responsible for all or even most language acquisition, she does claim that, under some conditions, CO facilitates second language learning in ways that differ from and enhance input due to the mental processes connected with the production of language. This hypothesis is closely related to the Noticing pyphthesis.
Swain defines three functions of output: 1. Noticing function: Learners encounter gaps between what they want to say and what they are able to say, and so they notice what they do not know or only know partially in this language. 2. Hypothesis-testing function: When a learner says something, there is always an at least tacit hypothesis underlying his or her utterance, e.g. about grammar. By uttering something, the learner tests this hypothesis and receives feedback from an interlocutor. This feedback enables reprocessing of the hypothesis if necessary. 3. Metalinguistic function: Learners reflect on the language they learn, and thereby the output enables them to control and internalize linguistic knowledge