It will go through the stages of the theory and some applicable strategies for L2 teachers and learns.Key words: mother tongue, second language acquisition, learning, L2, theory Linguist Stephen Krashen (1981,1982) , University of Southern California, USA has developed the most famous second language acquisition theory (SLA) which is also known as the Krashen’s Monitor Model.Fillmore and Snow, 2002 and Hamayan, 1990 stated that teachers can play a positive role in improving second language acquisition if they understood how to improve the learners’ ability of majority.
The role of filtering the new input increases as the learner have sufficient comprehensive input.
At later stage, when the second language learner masters a lot of L2 rules and have reached the advanced language proficiency, filtering becomes subconscious process.
This hypothesis in second language acquisition assumes that mastering second language grammatical rules occurred in a predictable order.
Regardless of the mother tongue of the learners, acquiring the target language rules varies in terms of their sequential which means that some rules are acquired earlier than others.
Krahsen has developed his theory of second language acquirers who are assumed to have two autonomous systems for improving their ability in acquiring a second language and aware of the language learning.
So, these systems are interconnected in an explicit way where unconscious acquisition seems to be more vital as it takes place naturally.
On the other hand, formal language learning is thought to be overwhelmed by a great deal of error correction and the existence of explicit grammatical rules (Krashen and Seliger, 1975).
Although error alteration is sustained, it helps the acquirer comes to the right psychological image of the linguistic simplification.
It is said that, error correction hinders the language development with a feeling of continues anxiety governed by error phobia .
Whether such feedback has this effect on the acquirer to a significant degree or not remains an open question (Fanselow, 1977; Long, 1977).