You’ve read your draft four times and it looks fine. By the fourth pass, your brain is filling in the comma that isn’t there and skipping the one that shouldn’t be.
A punctuation checker has no such problem. It applies the same rules to sentence one and sentence one hundred, which makes it excellent at some jobs and nearly useless at others. Some of what it flags are genuine mistakes. Others are style choices or settings, and the tool can’t tell the difference.
This article covers the seven flags you’ll see most often. Each one gets a before-and-after example and a rough trust rating. The ratings are my judgment based on how each rule works, not the result of a lab test.
In this article
- Comma splices
- Apostrophes
- Semicolons
- The Oxford comma
- Run-on sentences
- Quotation marks
- Exclamation points and em dashes
- What a checker still misses
- Try it on a practice paragraph
Quick reference
| Flag | Trust the checker? |
|---|---|
| Comma splice | Mostly yes |
| Its / it’s and similar apostrophe errors | Yes |
| Plural possessives (clients’ vs. client’s) | Check it yourself |
| Semicolon before a dependent clause or list | Yes |
| Whether a semicolon is the best choice | Your call |
| Oxford comma | Only for consistency |
| Fused sentences | Mostly yes |
| Quotation mark placement | Only with the right region set |
| Exclamation points and em dashes | Treat as a prompt, not a rule |
1. Comma splices
Trust level: mostly high.
A comma splice joins two complete sentences with nothing but a comma.
Before: The report was due Friday, nobody had started it.
After: The report was due Friday. Nobody had started it.
Checkers do well here because the pattern is easy to spot: a full clause, a comma, then another full clause. You have three ways out. Use a period, use a semicolon, or add a conjunction (“The report was due Friday, and nobody had started it”). The tool will propose one, but pick whichever fits the rhythm of your paragraph.
There’s one exception. Short, parallel clauses like “I came, I saw, I left” are a deliberate stylistic choice, and careful writers use them. The checker will flag them anyway, and you can ignore it.
2. Apostrophes
Trust level: high for the classic slips, low for plural possessives.
The usual offenders are its versus it’s, plurals written with an apostrophe (“photo’s”), and possessives with no apostrophe at all (“the dogs bowl”).
Before: The firm finished it’s audit of the clients accounts.
After: The firm finished its audit of the clients’ accounts.
The checker will catch it’s and notice the missing apostrophe in clients accounts. What it can’t do is decide where the apostrophe goes. Client’s and clients’ are both correct, and which one you need depends on whether you have one client or several. Only you know that. For plural possessives such as the writers’ room or the Joneses’ house, check the placement yourself every time.
3. Semicolons
Trust level: good at catching misuse, silent on whether a semicolon was the right call.
A semicolon joins two complete, closely related sentences. Checkers reliably catch the two most common misuses.
Before: Although it rained; we kept walking.
After: Although it rained, we kept walking.
Before: You’ll need three things; a laptop, a charger, and patience.
After: You’ll need three things: a laptop, a charger, and patience.
In the first, one side of the semicolon is an incomplete clause. In the second, a colon is the right mark for introducing a list. What a checker can’t judge is whether a correct semicolon is a good choice. A period is often cleaner. If you notice semicolons stacking up in your draft, reread those sentences even when nothing is flagged.
4. The Oxford comma
Trust level: reliable only as a consistency check.
The Oxford comma is the comma before “and” in a list. Neither version is an error. AP style leaves it out unless clarity demands it, and the Chicago Manual of Style puts it in. What matters is picking one and sticking to it.
Before: We sell pens, notebooks and staplers. Our support team handles returns, billing, and shipping.
After: We sell pens, notebooks, and staplers. Our support team handles returns, billing, and shipping.
Set your checker to follow your style guide. If it flags a list in a document where you’ve been consistent, that’s a settings mismatch, not a writing error.
There’s one case where the comma earns its place regardless of house style: when a list could be misread.
Ambiguous: I’d like to thank my sisters, the editor and the cat.
Clear: I’d like to thank my sisters, the editor, and the cat.
Without the second comma, “the editor and the cat” could read as a description of the sisters.
5. Run-on sentences
Trust level: mostly high, but it only catches missing punctuation.
A run-on, also called a fused sentence, runs two complete thoughts together with no punctuation or conjunction between them.
Before: The launch went well nobody complained.
After: The launch went well, and nobody complained.
Length is a separate issue. A long sentence that’s built correctly isn’t a run-on. If your checker flags one for length alone, that’s a readability warning, and it deserves a different kind of attention than a punctuation error.
6. Quotation marks
Trust level: depends on your region setting.
American style puts periods and commas inside the closing quotation mark, even when they aren’t part of the quoted words. British style places them inside only if they belong to the quotation.
US: She called the plan “ambitious.”
UK: She called the plan “ambitious”.
Colons and semicolons sit outside the quotation marks in both styles. Question marks and exclamation points go inside only when they’re part of the quoted text. Many British publishers also prefer single quotation marks, so check that too.
If your tool defaults to US rules and you write for a UK audience, you’ll get a stream of “errors” that are correct for your readers. Set the region before you trust any quotation flag.
7. Exclamation points and em dashes
Trust level: low. These are style prompts, not rule violations.
Grammar guides don’t set a maximum number of exclamation points or em dashes. Checkers flag them because heavy use tends to make writing sound breathless.
Before: Big news! Our app is faster! It’s simpler! Download it now!
After: Our app is now faster and simpler to use. Download the update today.
Em dashes work the same way. They’re useful for an aside or a sharp turn, but once every paragraph has one, they become a habit rather than a choice. Treat the flag as a nudge to reread, not an order to delete.
What a punctuation checker still misses
A checker works from patterns, so it struggles wherever the right answer depends on what you meant.
Commas that change the meaning. “I’m meeting my brother, Tom, for lunch” implies you have one brother. “I’m meeting my brother Tom for lunch” implies you have more than one. Both are grammatical, and no tool knows your family.
Valid sentences that say the wrong thing. “The drivers who were late were fined” and “The drivers, who were late, were fined” are both correct. The first fines only the late drivers, and the second says all of them were late and all were fined.
Hyphens. A “man eating shark” and a “man-eating shark” are very different animals. Compound-modifier hyphens are easy to get wrong and easy for a tool to miss.
Commas after introductory phrases. Many style guides treat the comma after a short opener (“Last year we moved”) as optional, and checkers disagree on whether to flag it. Follow your style guide rather than the tool.
Try it on a practice paragraph
The best way to learn which flags to trust is to test a checker yourself. Paste this paragraph into yours. It contains seven planted problems, written with a US style and the Chicago-style Oxford comma in mind.
Our new office opens Monday, everyone is excited. The kitchen has it’s own coffee bar; although nobody has figured out the machine yet. The parking lot is already full nobody can find a spot. Bring three things: your badge, your laptop and a sense of humor. Lunch will be pizza, salad, and fruit. The CEO called the move “overdue”, and the manager’s are relieved!
Answer key
- “Monday, everyone” is a comma splice.
- “it’s own” should be “its own.”
- The semicolon before “although” should be a comma.
- “full nobody” is a fused sentence and needs punctuation or a conjunction.
- The two lists are inconsistent: one has the Oxford comma and the other doesn’t.
- In US style, the comma belongs inside the quotation marks: “overdue,”.
- “manager’s” should be “managers” (a plural, not a possessive).
The exclamation point at the end is a style call. Score your checker on how many of the seven it caught, how many it flagged wrongly, and how many it missed.
What to do next
Think of a punctuation checker as a second pair of eyes that is strong on mechanics and weak on judgment. Comma splices, its/it’s, and misused semicolons are safe to accept. The Oxford comma, quotation placement, and exclamation points are settings and style decisions that belong to you.
Run one real draft through it today, and for each flag ask whether it’s a rule or a preference. You’ll quickly learn which flags you can click through and which deserve a second look.
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