According to Stephen Krashen (2007), language acquisition occurs when messages are comprehensible and when “speakers are concerned not with the form of their utterances, but with the messages they are conveying and understanding.” Krashen is an expert in linguistics and second language acquisition. He is well-known for his five hypotheses about second language acquisition.
“Krashen’s theory of second language acquisition claims that second languages are (1) acquired, not learned. The process is the same as for first language acquisition. Acquisition occurs in a (2) natural order when people receive (3) comprehensible input and their (4) affective filter is low. Rules that people learn can be used to (5) monitor the output, either speech or writing. Krashen’s theory of second language acquisition provides the theoretical base for content-based language teaching.”
Freeman, D. and Freeman, Y. English Language Learners The Essential Guide, (2007)