The clear understanding of the differences between acquisition and learning makes it possible to investigate their interrelationships as well as the implications for the teaching of languages.
First, we ought to consider that languages are complex, arbitrary, irregular phenomena, full of ambiguities, in constant random and uncontrollable evolution. Therefore, the grammatical structure of a language is too complex and abstract to be categorized and defined by rules.
Even if some partial knowledge of the functioning of the language is reached, it is not easily transformed into communication skills. What happens in fact is the opposite: to understand the functioning of a language with its irregularities is a result of being familiar with it. Rules and exceptions will make sense and grammar, word choice and pronunciation will be employed appropriately if it "sounds" right. Language analysis and the deductive, rule-driven study of grammar are not only ineffective to produce communicative ability, but also frustrating. It is much easier and more enjoyable to acquire a language than it is to learn a language.
In his Monitor Hypothesis Krashen admits that the knowledge obtained through formal study (language learning) can serve to monitor speaking. Krashen, however, doesn't specify the language that would be the object of study, but it is logical to assume that he was using the study of Spanish as the basis for his inferences and conclusions because it is the dominant foreign language in the United States, and particularly in the state of California, where Professor Krashen lives and works.
Therefore, it is necessary to analyze the characteristics of the target language, their degrees of irregularity and difficulty and how that affects the applicability of Krashen's theory. It is also necessary to analyze the personal characteristics of the players in the teaching-learning arena.