In second language acquisition research conducted in 1974-75, 1980 and 1987, it was postulated that the acquisition of grammatical forms followed a natural and predictable order. How this happens is contingent upon multiple factors. The learner's age and the learner's circumstances seemed not to be a significant influence on this natural order. Dr. Krashen makes the point that this does not mean some sort of curriculum should be devised based on this order.
Krashen's entire point seems to be that there is a difference between the conscious learning of grammatical structures and the unconscious acquisition of speech, no matter the language. Acquisition of speech is far more important in the empowerment of someone who wants to speak the language?spoken fluency.
Critics would say Krashen has drawn too rigid a line between the learning and the acquiring of a second language. Some believe Krashen made these distinctions based on a specific or particular environment in which the learners were found and did not consider the classroom might be an environment that would have some importance in second language acquisition.
Krashen's critics do agree with him that the mass teaching of a linguist's approach to cold, grammatical principles does not facilitate second language acquisition. They go on to suggest that within a classroom, it would be best to help the student construct his own grammar so that he might reach full mastery of the language.
This seems to me to be a throwback to some earlier approaches in which the student, through some kind of touchy-feely existential probing, comes up with "his own grammar."
A natural order might, on the other hand, emerge in the process in which the child, or the second language learner, hears hundreds upon thousands of repetitions from those within the learner's environment who speak the language with correct form and structure. This situation is where the unconscious assimilation of correctness comes. Language is then acquired!