Students need to learn the principles of grammar in order to communicate effectively. This is an argument that even the grooviest of descriptive linguists has a hard time disagreeing with. In order to be able to break the rules in ways that are aesthetically or rhetorically effective, you first have to master them. But the same, I humbly argue, applies if you are going to be ultra-picky about grammar.
I am speaking especially to instructors who shave points off papers for perceived grammatical mistakes, particularly in classes where you are not actually teaching writing. In order for that sort of system to be fair, it needs to be pretty nigh close to infallible, but unfortunately, in my years working in writing centers and as an independent writing coach, I’ve seen plenty of papers on which mistakes are marked that aren’t actual mistakes. There’s the egregious stuff, like the business communication instructor who confuses passive voice with intransitive verbs (no, really). And there’s the subtle, but arguably more insidious stuff, like the marking of things as wrong that are not in fact grammatically wrong could, perhaps, benefit from editing to deal with wordiness, vagueness, or syntactical awkwardness. I’ve seen sentences like this receive nothing but an “X” with “GRAMMAR” in the margin without any guidance as to what rule the student has broken or how they should correct it. It’s notations like these that indicate to me that the instructor in question hasn’t the foggiest idea what they’re actually talking about when they talk about grammar.
The truth is that effective editing is often a subjective process that involves not the upholding or breaking of set-in-stone rules but selection among multiple legitimate choices depending upon what it is the writer wishes to communicate. But acknowledging that questions of what makes effective writing are at bottom so subjective is threatening both to instructors and to students who are looking for something concrete upon which to base a grade. Sure, you can point to really specific things like comma splices and sentence fragments, but when it gets down to more complex writing issues, students will invariably find that their grade depends upon the hobby horses of their specific teachers. This one says the passive voice is always wrong. This one says never begin a sentence with a conjunction. In this context, such attempts to present grammar as a set of absolute, inarguable statutes, students receive the message that they are going to be punished for doing things they didn’t even know were wrong. That they may, in fact, be punished for doing something in X’s class that was considered correct in Y’s class. Last year, I had a graduate student bring me her marked paper, distraught by the points she lost for grammar, and I looked through the marks and had to tell her that honestly, in some places she lost points for things that weren’t wrong. And in fact, in at least one case, her instructor had changed a verb tense that was correct originally but now no longer agreed with the subject.
This is unacceptable. If you are going to make an issue out of grammar in your class, you need to be prepared to teach grammar in some form or another, even if that only means making resources readily available for students who exhibit patterns of error. But the bottom line remains that you, yourself need to know what you are talking about.
The same thing goes if you are flitting about the internet correcting people’s tweets or interrupting casual conversation to comment on adverb usage. This is already socially marginal behavior, so don’t take the risk of both being a dick and a fool.