In his influential text Principles and practice in second language acquisition (1982) Stephen Krashen compiled all of his hypotheses in second language learning under the "Monitor Model". The "Monitor Model" includes
a) the affective filter theory- which states that a "full" filter (an overall affect of stress and frustration) impede the learning of L2.
b) natural order hypothesis- states that, when analuzed at a very close level, all learners in L1 and L2 acquire their L1 and L2 in a predictable pattern which repeats itself. While the L1 is not structurally similar to the L2, the ways in which they are acquired seem to follow the same neural pattern.
c) monitor hypothesis- contends that, rather than using language losely in verbal communication, L2 users tend to think before speaking in order to apply the proper syntactic rules that apply to the target language in which they are trying to communicate.
d) input hypothesis- states that, in order to develop L2 skills, we cannot merely hear the same words over and over in the target language, but that instead the i +1 format should be used. What this means is that, during each language exposure, the previously learned language must be pushed forward and become more challenged by the inclusion of more, and more complex words. It resembles the idea of a Zone of Proximal development in which a student is pushed to the next level.
e) acquisition-learning hypothesis- this hypothesis differentiates the process of learning from the process of acquiring language. Learning is different from acquiring because learning is a conscious effort to retain information. Acquisition is the indirect and natural process of allowing information to flow subconsciously through processes such as playing, singing, or interacting.
The Monitor Model is basically the "go-to" place to comprehensively understand the basic concepts of Krashen's most popular hypotheses.