M4: Krashen -- Acquisition vs. Learning & Monitor Hypothesis
Krashen's theory of acquisition vs. learning is about HOW we might become proficient in a new language. The left (acquisition) is good, the right (learning) is not as good.
We want our ESL classes to be as informal and play-like as possible so that our students truly acquire more language. If all we do is drill repetitive language structures and teach grammar, students are not as likely to acquire the new language.
Another theory of Krashen's is the Monitor Hypothesis.
This theory makes sense because the better you get at language (the more proficient you are) the more you can monitor your own output -- you know if what you said is correct or not. If it is incorrect, you can correct it either before you say it or right after (or you can correct your writing).