I once spent ten minutes arguing with a grammar checker over a sentence that was completely fine. It kept underlining "data is" in red, insisting it should be "data are," like I'd made some rookie mistake. I hadn't. I'd just bumped into one of those grammar rules that's a lot blurrier than any checker wants to admit.
If you've ever stared at a squiggly red line under a sentence that reads perfectly fine to you, you're not losing your mind. Sometimes the checker is wrong, or more accurately, it's applying a rule that isn't as fixed as it's pretending to be.
The "data is" versus "data are" trap
Technically, "data" is the plural of "datum," so old-school grammar says it should always take a plural verb, as in "the data are clear." But almost nobody talks that way anymore outside of academic writing, and most major style guides, including the Associated Press, have shifted to treating "data" as a collective singular noun in everyday use. "The data is clear" is now completely standard in most writing.
Some checkers haven't caught up to that shift, or they're erring on the side of the stricter historical rule. Either way, you're not wrong to write it the modern way, you've just run into a rule that's still in transition.
Starting a sentence with "And" or "But"
This one gets flagged constantly, and it shouldn't. There's a popular myth that you can never start a sentence with a conjunction like "and," "but," or "because." It's not a real rule. It never was. It's something a lot of us got told in school as a simplified way to stop kids from writing fragmented, choppy sentences, and it calcified into a "rule" that stuck around way longer than it should have.
Professional writers do this all the time, in fiction, journalism, and even formal essays, when it serves the rhythm of the sentence. If a grammar checker flags it, you can usually just ignore the suggestion. The actual rule is about clarity, not about which word a sentence is allowed to start with.
A lot of grammar advice isn't actually grammar. It's style preference dressed up as a rule.
Splitting infinitives
"To boldly go where no one has gone before" is one of the most famous sentences in pop culture, and it splits an infinitive right down the middle, sticking "boldly" between "to" and "go." Old grammar guides treated this as an error, borrowing the rule from Latin, where you literally can't split an infinitive because it's a single word. English doesn't have that limitation, and linguists have been saying for decades that the "rule" against splitting infinitives was never really suited to English in the first place.
Most modern checkers have backed off this one, but you'll still occasionally see it flagged, especially in older or more conservative tools. If avoiding the split makes your sentence sound awkward, don't force it. Natural phrasing wins.
So when should you actually trust the red squiggle?
- Subject-verb agreement errors, like "the list of items were missing" instead of "was missing"
- Genuinely missing punctuation that changes meaning, not just stylistic comma preferences
- Misused homophones, like "their" versus "there" versus "they're"
- Run-on sentences that are actually hard to follow, not just long
The pattern across all of this is pretty simple. Grammar checkers are great at catching mechanical mistakes, the kind where a sentence is objectively broken. They're a lot less reliable when it comes to style conventions that have shifted over time or were never universally agreed on to begin with.
How to actually use a grammar checker well
Treat every flag as a question, not a verdict. When something gets underlined, ask yourself whether it's a real error or just a style choice the tool happens to prefer. If you genuinely don't know, that's worth running through a checker to get a second opinion, especially before sending something important like a cover letter or a client email.
Just remember that the goal isn't a zero-flag document. The goal is writing that's clear and reads the way you intended. Sometimes that means keeping the sentence the checker didn't like.
- Some grammar "rules" are really just style preferences that calcified over time
- "Data is" has become standard in modern usage despite older guides preferring "data are"
- Starting a sentence with "and" or "but" is not actually a grammar error
- Splitting infinitives is a rule borrowed from Latin that doesn't really apply to English
- Grammar checkers are most reliable for mechanical errors, less so for style conventions
What to do next time you see a red squiggle
- Read the flagged sentence out loud and ask if it sounds natural
- Check whether the issue is a real error or a style preference
- If you're not sure, run it through a grammar checker for a second opinion
- Trust your own ear when the suggestion makes the sentence sound worse, not better