The Grammar Mistakes Spell Check Won't Catch
Spell check has trained a lot of people to believe their writing is clean the moment the red squiggly lines disappear. It isn't the same thing as grammar being correct, and the gap between the two is exactly where embarrassing mistakes survive all the way to a sent email or a published post.
This covers the kinds of errors that slip past basic spell checking, where tone and context trip up automated tools specifically, and where a grammar checker is genuinely useful versus where it needs a human reading it back afterward.
Why Spell Check Lets Real Mistakes Through
Spell check only catches words that don't exist. It has nothing to say about a sentence that's grammatically broken but spelled correctly, like using "their" instead of "there," or "its" instead of "it's." Both are real words, so the red squiggly line never appears, even though the sentence is wrong.
The Errors That Sound Right But Aren't
Some mistakes survive because they sound natural out loud. Subject-verb agreement breaks down in long sentences, where the subject and verb end up separated by enough other words that the ear stops tracking which one matters: "the list of items on the table were moved" sounds fine, but "list" is singular, so it should be "was." Misplaced modifiers do something similar; "I only ate breakfast" and "I ate only breakfast" mean different things, and most writers default to whichever one comes out first instead of the one they actually meant.
Tone and Setting Are Where Automated Tools Struggle
A grammar tool can confirm a sentence is structurally correct without knowing it's the wrong sentence for the moment. A casual email to a coworker doesn't need the same formality as a cover letter, and a tool that corrects both toward the same neutral, formal register can flatten a message until it sounds like nobody wrote it. The same goes for idioms: a tool that takes "it's raining cats and dogs" literally and tries to fix the grammar of a nonsense phrase is solving a problem that doesn't exist.
A Few Patterns Worth Learning by Hand
- ✓"Affect" is usually the verb, "effect" is usually the noun: a decision can affect the outcome, or have an effect on it.
- ✓"Who" refers to the subject of a sentence, "whom" to the object: a tool that flags every "whom" as wrong because it sounds formal is often flagging correct usage.
- ✓A comma splice joins two complete sentences with just a comma instead of a period, semicolon, or conjunction, and it's one of the most common errors that survives a quick read-through because both halves sound fine on their own.
When Breaking the Rules Is the Right Call
Fragments, sentence-starting conjunctions, and the occasional run-on aren't always mistakes. A short fragment can land harder than a complete sentence in a piece of persuasive writing, and dialogue in fiction routinely breaks formal grammar on purpose to sound like an actual person talking. A tool that fixes every fragment back into a complete sentence is optimizing for correctness in a context that actually wants something else.
Letting a Checker Catch the Obvious Stuff First
A grammar checker is genuinely useful for the mechanical layer: typos, subject-verb agreement, basic punctuation, the kind of error that's just a slip rather than a choice. Run it before anything goes out the door, but read the result yourself afterward, since it can't tell formal writing from casual writing, and it can't tell you when a broken sentence was the right call. For deeper style and usage questions beyond what any checker covers, the Purdue Online Writing Lab is a reliable, free reference.
If you're cleaning up a draft rather than catching errors in a finished one, our paraphraser is built for rewriting awkward phrasing rather than just flagging it. For a closer look at where AI grammar tools tend to fall short, see our breakdown of the truth about AI grammar checkers.
Frequently Asked Questions
Why didn't spell check catch an obvious grammar mistake?▼
Spell check only flags words that don't exist in the dictionary. A sentence can be grammatically wrong while using entirely real, correctly spelled words, like confusing your and you're, which spell check has no way to catch.
What's the difference between a comma splice and a run-on sentence?▼
A comma splice joins two complete sentences with only a comma. A run-on sentence joins them with no punctuation at all. Both can usually be fixed the same way: split into two sentences, or connect them with a semicolon or a conjunction.
Should I trust an automated grammar checker for formal writing?▼
Use it as a first pass for mechanical errors, then read the result yourself. Automated tools are reliable for catching typos and basic agreement errors, but they can't judge whether the tone fits the situation, which matters more in formal writing than grammar alone.
Is it ever correct to start a sentence with and or but?▼
Yes. It's a stylistic choice, not a grammar error, and it's common in published writing when a short sentence needs to land with more punch. Most rules against it are style preferences passed down as if they were grammar rules.
Can grammar tools handle idioms and figurative language?▼
Not reliably. Idioms often look like nonsense or broken grammar if read literally, and a tool that tries to correct the words in a phrase like it's raining cats and dogs is solving a problem that isn't there.