You learn proper usage mostly by being well-read. This is why reading matters so much; learning grammar rules only gets you so far, especially in a language where every rule has twice as many exceptions.
Explain and drill
Explain the difference, have 10 examples where the student has to cross out one or the other option then the student has to create 3 examples of each of their own - depending on the ability of the class this could be either totally free choice, or they are given sets of clauses that they need to approprately join, (maybe by adding more than just then/than to make a “good” sentence.
ETA - given that I made an error that I have corrected maybe they could also spot and correct the mistake in a sentence or story as well
And a second edit - this is the sort of exercise that AI is particularly good for, this sort of worksheet can be created in a few minutes (although it does need to be proofed) instead of making the teacher crack brain
The vast majority of what I know about the English language comes from reading it, hearing it, and using it. Context can be a great teacher.
I can’t tell you how many times I see something written and think to myself that something isn’t right without being able to properly articulate what is wrong at first, and certainly not being able to cite a formal “rule” about it. It becomes something instinctual after you have enough familiarity with the language. And yeah, reading a lot helps in this immensely.
I can’t even remember being taught formal grammar. What I remember is reading voraciously. So if something is grammatically incorrect, it hits me a certain way, and I know it’s wrong, even if I don’t know why.
There are a few exceptions where I can’t seem to learn them even when I’ve been taught the rule. Who/whom is a cluster, you and me/you and I, which vs. that… I have zero confidence in those. And it’s not grammatical but any word with double consonants in some places but not in other places really messes me up.
I can still remember the exact moment I learned the distinction: while reading David Schwartz’s How Much Is a Million at age 7, I encountered the word “than” and ran off to excitedly tell my parents I’d found a spelling error in the book. Imagine my surprise….
I was taught their/they’re/there; when to use “I” vs “me” as in “Joe and I went…” and “Give it to Joe and me”; effect vs affect. I received the lessons several times between grade school and graduation, and had it all down as second nature a few years before I started reading grownup material. My head explodes a little bit whenever someone uses “I” as the object of a preposition.