The requirements for language acquisition differ, yet the three theories can be viewed on one continuum, locating L2 comprehension on one end and L2 production on the other.
Comprehensible input is the necessary but also sufficient condition for language acquisition to take place. It requires no effort on the part of the learner.
Learners use their existing acquired linguistic competence together with their general world knowledge to make sense of the messages they receive in language just beyond where they currently are (the +1).
This theory has clear implications for language teachers; namely that their language instruction should be full of rich input (both spoken and written language) that is roughly tuned at the appropriate level for the learners in the class.
Gass & Varonis 1994) Negotiations crucially focus the learner’s attention on the parts of the discourse that are problematic, either from a productive or a receptive point of view. Attention in turn is what allows learners to notice a gap between what they produce/know and what is produced by speakers of the L2. The perception of a gap or mismatch may lead to grammar restructuring.
This is an important aspect of Vygotsky’s theory. It is “the idea that the potential for cognitive development is limited to a certain gap, which he calls the zone of proximal development (ZPD). Learning during the ZPD depends upon full social interaction. Vygotsky felt that the range of skills that would be developed with teacher guidance or through peer collaboration would exceed those that might be achieved by a learner working alone” (Beatty, 2010, p. 104).
Negotiation of meaning is a collaborative back and forth process that speakers go through to reach a clear understanding of each other’s message in spite of communication difficulties.
Swain (1985) argues that Krashen’s (1985) Input Hypothesis is insufficient for L2
acquisition, and that opportunities to produce comprehensible output are also necessary.
These are typically formed by tag questions, by repetitions of all or part of the teacher's preceding utterance(s) uttered with rising question intonation
Any expression by the teacher immediately following an utterance by the student which was designed to elicit confirmation that the utterance had been correctly understood or correctly heard by the teacher
requests “indicate to students either that their utterance has been misunderstood by the teacher or that the utterance is ill-formed in some way and that a repetition or a reformulation is required.”