My students taught me the most valuable lessons I learned as an ESL teacher — my students, not any book or professor or training or university. Everything I learned about teaching, I learned from my interactions with my students.
For example, during my earlier years as an ESL teacher, decades ago, I learned never to use "never" (or "always") when teaching grammar.
One day, while teaching adjective clauses, specifically restrictive versus descriptive clauses, I told my students that we never put a comma before the relative pronoun "that" in adjective clauses.
At the next class meeting, someone brought me a sentence with a comma before "that" and asked me why a comma appeared before the word "that" in the grammar book. I immediately understood the student's confusion and explained that what I said was about adjective clauses and didn't apply to the sentence he had brought to my attention in the grammar book because it was not an adjective clause — it was a noun clause connector in a series separated by commas.
Another time, during a lesson on modal verbs, after teaching could have, should have, and would have, I told them that the "have" may sound like "of" and explained that these get misspelled quite often because of how they sound: could of, should of, and would of... even by American writers.
It's important to teach ESL students the correct structure and warn them about what they might encounter (just so they don't doubt themselves and their own abilities, which can easily happen when they are faced with a person they believe should master the language).
Shortly after I taught my students to pay attention to the spelling of past modals and that it's never could "of," I came across a correct sentence with "... could of..." and thought of my students and the lesson I had just taught them. I don't remember what that sentence was years ago, but I recently came across another sentence like that, which brought this memory back.
That day all those years ago, I learned to never use "never" or "always" in my grammar classes. Why? Because some students may lack the knowledge to distinguish structures, clauses, etc., and the misunderstanding created as a result of this might confuse the students or make them lose their trust in their teacher, which can be extremely frustrating for anyone trying to make sense of a new language.
In case you're wondering what the sentence was, here's the one I recently saw while editing a book:
"He reached down, picked up what he could of the brown bag, and walked on home."
(Of course, the could and the of are not "together" (meaning they don't form a phrase), but the two words are close enough in the sentence to confuse English learners who have just learned modal verbs.)
