People love to write about writing. I edit such articles frequently.
All too often, these pieces are titled "10 essential grammar tips" or the like, and the author proceeds to talk about common errors in writing that aren't actually grammatical in nature, or he or she compares anvils and oranges.
My friend and cohort Mignon Fogarty once said to me, "I'm not really Grammar Girl; I'm more like Usage Girl." It's true; she devotes much of her time and energy to offering advice on word choices and meanings. The alliteration of Grammar Girl is catchy, and she's terrific, so why argue with success?
With that in mind, let's differentiate the various elements of writing—with the goal of preventing writing "experts" from calling a common typo or misused hyphen a "ubiquitous grammatical error."
Yes, technically they are misspellings, but they usually involve dropped, extraneous, or transposed letters resulting from either fat fingers or fast fingers. (Case in point, I typed fasat fingers just now.) Some examples: avergae instead of average; you when your is meant; that when than is intended; wither in lieu of either. (These are all from my personal vault; your typos may vary.)
Homonyms/homophones are a prime culprit here: to/two/too, they're/there/their, you're/your, and so on. Other misspellings seem born out of simply not knowing how to spell a given word: guerilla instead of guerrilla (spell-check didn't even flag the former—how sad); milenial instead of millennial; embarassment in place of embarrassment. Of course, the -ible and -able endings frequently wreak havoc.
Certain misspellings arise from mishearing: Those who have always heard melted sugar called carmel will spell it that way, rather than the correct—and far more delectable—caramel. (This is a regional thing, it seems, and it makes me nuts. Then again, nuts and caramel are delicious together.)
This is to say nothing of made-up words, such as supposably—as in the following sentence, "Phil, if you say 'supposably' again, I'll slug you."
This is a lesson for another day, Grasshopper. Certainly bad punctuation can muck up one's writing, even skewing the grammatical structure of a sentence. However, if you're setting out to give a primer on grammatical pitfalls, it's better to set aside your homily on whether commas and periods belong inside or outside quotation marks.
This is about applying words for their proper meaning. Many writers seem to think that employing a less-common form of a word somehow adds emphasis: penultimate, incredulous. Penultimate means next-to-last, not exceedingly ultimate (whatever that might be), and incredulous means disbelieving, not extra incredible (ditto). [Author's note: Some dictionaries have included vernacular uses and misuses of these and other words as secondary and tertiary entries. I don't care. Please sound better informed than that. Thanks.]
During means for the duration of. "During the eulogy, Aloysius wiped a tear from his eye." Must've been one helluva tear; old Aloysius spoke for 45 minutes.
Momentarily means for a moment, not in a moment. Remember it this way: There will be a momentary delay. The delay will last a moment.
It's understandable why "grammar" has become the catch-all for verbal issues and elements. Grammatical rules provide the framework for our understanding the message: "Dog bites man," versus, "Man bites dog." Subject, verb, object. Without specific noun cases, such as German has, our word order makes all the difference.
It's impossible to cover every aspect of such a broad topic, but I would be remiss in not noting an increasingly common flaw in today's writing—the lack of subject/verb agreement, usually with a compound subject. Take this: "Humor and compassion is essential to success as a stonecutter." Nope. The plural subject, Humor and compassion, demands a plural verb form: are. Similarly, one I just stumbled upon: "Here's the lessons I learned." Are. Here are …
This might be the trickiest aspect to understand and/or convey. If grammar is the basic structure, syntax is the nuanced juxtaposition of words and phrases.
"Not all birds are eagles," is different from, "All birds are not eagles." (The latter's not quite true, is it?)
"The senator said last month that she'd introduced the legislation," or, "The senator said that she'd introduced the legislation last month." (The time element is specified on either the saying or the introducing.)
"Maurice says he wants to paint only the kitchen," or, "Maurice says he wants only to paint the kitchen," or, "Only Maurice says he wants to paint the kitchen." (Here the emphasis is on  what will be painted,  a limit on the work to be done, and  who's willing do to the job.) Then again, "Maurice only says he wants to paint the kitchen." (Come on, he'll weasel out of it.)
The word where too often is used in instances that call for when or in which.
"I had a blog post yesterday where I explained…" This has the aforementioned wrong type of modifier, but it's also badly located. How about this: "Yesterday I had a blog post in which I explained…"
Better still: "In yesterday's blog post, I explained…"
This applies to whether you write omelette or omelet, whether you call him President Obama or President Barack Obama on first reference, whether you use % or percent or per cent, whether you prefer toward over towards, and whether you use the Oxford comma in every series.
This is another topic that could occupy an entire article, but let's render this in one simple example: If you want to say $100 billion and you write $100 million instead, you've booted the accuracy quotient-and probably some credibility.
Let me stress that in this article I'm aiming simply to delineate these elements, not to provide a comprehensive treatise on verbal stink bombs. Let's hear from you, though, dear readers. Do you see articles that blur the lines between these elements? Please sound off in the comments section.