Stephen Krashen is a very famous linguist who has written theories about second language acquisition. Here is a summary of something he said that is very controversial:
The hypotheses put primary importance on the comprehensible input (CI) that language learners are exposed to. Understanding spoken and written language input is seen as the only mechanism that results in the increase of underlying linguistic competence, and language output is not seen as having any effect on learners' ability. Furthermore, Krashen claimed that linguistic competence is only advanced when language is subconsciously acquired, and that conscious learning cannot be used as a source of spontaneous language production. Finally, learning is seen to be heavily dependent on the mood of the learner, with learning being impaired if the learner is under stress or does not want to learn the language.
What he is saying is that doing things like memorizing vocabulary and studying grammar won't improve your language skills. He calls things like these "Learning", and says language is "acquired", not "learned". Do you agree and why?