Since the 1980s, Stephan Krashen’s Theory has become very acknowledged and generally accepted. His theories have greatly impacted both bilingual education and teaching as well as further research.
Dr. Krashen’s Theory of SLA including a collection of five main hypotheses: the Input Hypothesis, Acquisition-Learning Hypothesis, Monitor Theory, the “Affective Filter”, and the Natural Order Hypothesis.
Dr. Krashen proposes that individuals have two distinct methods to develop basic proficiency in any particular language: language acquisition versus language learning.
In contrast to learning, language acquisition is a subconscious process, similar to the way we all learned our first language by listening to others (commonly referred to as “picking up”). Like children, language acquirers are not aware of the grammatical rules and nuances, but instead developing an "intuitive idea" for correctness. Language learning focuses on the conscious knowledge of a particular language instead.
The Acquisition-Learning Hypothesis distinguishes that contrary to popular belief, adults do not lose the ability to acquire languages (analogous to “picking up” a particular language).
The monitoring function is the result of the learned grammar. The role is to correct deviations from “normal” speech and give speech a more “polished” appearance.
The central idea of the Natural Order Hypothesis is that "the acquisition of grammatical structure occurs in a specific predictable order." For any language, certain grammatical structures tend to be acquired first, and others may be late, regardless of a speaker’s native language. This suggests that grammar may not necessarily need to be taught in this natural order of acquisition.
The Input hypothesis explains the process in which the learner acquires a second language. This hypothesis is solely concerned with “acquisition”, not “learning” (in any sense of the word). According to the Input hypothesis, the learner progresses following the sequence of “natural order” until he 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 SLA will take place when one is exposed to “Comprehensible Input” that belongs to “Stage II.” As a result, Dr. Krashen proposes that natural communicative input is the core of SLA, ensuring in this way that each learner will receive some “Stage II” input that is appropriate for one’s current level of linguistic skill.
The affective filter is a number of “affective variables” that play a facilitative role in second language acquisition. They include: motivation, self-confidence and anxiety. Learners with high motivation, confidence, 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 “raise” the affective filter and form a “mental block” that can impede language acquisition.