A spell checker that only knows words would pass Their going to send you're order without a mark, because every word in it exists. A grammar checker looks at how the words fit together, and that is where most everyday writing mistakes are.
Here are ten mistakes a rule-based grammar checker is built to recognize, each with an example and its fix. None of them says anything about how well someone writes. They are the slips everyone makes when typing fast, and the reason to let a checker read along.
1. A verb that does not agree with its subject (subject-verb agreement errors)
A singular subject takes a singular verb. The mistake happens when other words stand between the two, and the verb agrees with the nearest noun instead of the real subject.
- Before: The list of changes are attached.
- After: The list of changes is attached.
2. Its and it's (its vs. it's)
It's is short for it is or it has. Its means belonging to it, and like his and hers it has no apostrophe. If you can replace the word with it is, write it's.
- Before: The app saves it's settings automatically.
- After: The app saves its settings automatically.
3. Their, there and they're (their vs. there vs. they're)
Their shows ownership, there is a place or starts a sentence like there is, and they're is short for they are. All three sound the same, which is exactly why fingers mix them up.
- Before: Their going to call us tomorrow.
- After: They're going to call us tomorrow.
4. Your and you're (your vs. you're)
The same pattern: you're is you are, your is something that belongs to you.
- Before: Thank you for your order, your welcome to contact us.
- After: Thank you for your order. You're welcome to contact us.
5. Then and than (then vs. than)
Than compares, then tells you when. After a comparative such as better, faster or more, the word is than.
- Before: The new version is faster then the old one.
- After: The new version is faster than the old one.
6. A or an (a vs. an)
The choice depends on the sound that follows, not the letter. An goes before a vowel sound, a before a consonant sound. So it is an hour (a silent h) but a user and a one-time code, which start with a consonant sound.
- Before: It took a hour to create an user account.
- After: It took an hour to create a user account.
7. The same word twice (repeated words)
A doubled word is almost invisible to its author, especially across a line break, because the brain reads the sentence it expects. A checker has no expectations and spots it at once.
- Before: Please send the the report by Friday.
- After: Please send the report by Friday.
8. No comma after an introductory word
Words such as however, unfortunately and meanwhile at the start of a sentence are followed by a comma. Without it, the reader stumbles over where the sentence really begins.
- Before: However we decided to wait until next week.
- After: However, we decided to wait until next week.
9. Could of, should of, would of (could have, should have, would have)
Spoken quickly, could've sounds like could of, and that is how it ends up written. The correct form is always could have, should have or would have, or the short form with an apostrophe.
- Before: We should of checked the figures first.
- After: We should have checked the figures first.
10. A missing capital letter and an extra space
A sentence starts with a capital letter, and punctuation sits directly after the word it belongs to, with a space after it rather than before. These slips come from quick typing and from text pasted together from several places, and they make a careful message look rushed.
- Before: The file is attached . please check it before Monday.
- After: The file is attached. Please check it before Monday.
What a grammar checker can miss in your writing
Rules recognize patterns. Some mistakes do not form a pattern a rule can name:
- A correct word in the wrong place. Please sign the attached contact is a correct sentence, and so is Please sign the attached contract. Only you know which one you meant.
- Two sentences joined by a comma. The file is ready, you can download it now is a comma splice. Whether it is acceptable depends on style, so checkers treat it with care.
- Meaning. No grammar rule knows that the meeting is on Thursday, not Tuesday, or that the total should be higher.
For mistakes that depend on context, an AI spell checker that reads whole sentences can help; Spell checker vs AI spell checker: which should you use? explains the difference. For everything no software can know, use A proofreading checklist before you hit send.
Try the grammar checker on your own text
Paste a recent email or a paragraph of a document into the Online Spell Checker, choose your variant of English, and press Check spelling. Every mark is explained beside the text, so you see not only what to change but why, and the next time you type it's you may stop to think about it first.