I suspect that many students do not learn much about English grammar because their English teachers do not know much about English grammar. Grammar has too many exceptions to too many rules which make it a little too likely that the teachers will be caught in an embarrassing moment where they do not know the answer to a seemingly simple question. The solution becomes ignoring the topic all together or treating it in a cursory manner while making up ridiculous “rules.”
The “rule” that probably does the most damage is the “put-a-comma-whenever-you-hear-a-pause” rule. I hate this rule because it is not only wrong, but it teaches students that grammar is intuitive rather than logical. A bigger problem is that this rule sticks. Students forget just about everything else they hear about grammar, but they remember this “rule.” It’s simple, and it seems to make sense.
The problem is that it makes no sense if you know anything about grammar. It makes the logic of grammar into an arbitrary set of “rules” that can be applied in any way the writer seems to think makes sense. One writer might pause here while another might pause there, and no attention is given to the structure of the language. In the backs of their minds, I suspect that students know this “rule” is meaningless, but pushing their teachers on the subject would make for a longer day and a more complicated lesson.
Students end up internalizing this meaningless “rule” and examine their sentences for these pauses that might or might not be there. The result is a fairly random approach to using commas and a good deal of frustration about writing.
What seems to be happening is that students are learning to confuse an approach they should be taking to style with an approach they are taking to grammar. Style should avoid giving absolute rules and should rely on the intuition of the writer. Grammar should not. It needs to be logical, and students need to recognize that logic in the their own writing and the writing of others.