According to Krashen, there are two systems of foreign language performance: the affective system and the formal system. One is the product of formal and conscious knowledge that we manipulate in the communicative act. The other is the result of our emotions and feelings, which affect our social position through communication.

The affective filter hypothesis explains the relationship between the form of learning a language and our communication system. Essentially, it is the grammatical filter that is always in play while we communicate.

It is the way by which Krashen explains the process of acquiring a second language vocabulary. According to this hypothesis, one improves one's lexical knowledge by receiving second language input from someone they are effectively related to.

It is the conception that second language acquisition goes through three stages. The first one with the introduction of a small lexicon. The second, with the appearance of morphology and complex syntax. And the third and last one with the affective input (i.e. performance aspects) in language usage.

It is the conception that affective variables play a facilitating role in the process of acquiring a second language. Krashen argues that learners who have high self-confidence, a high motivation and have a good self-image are better equipped for success in second language acquisition.