📋 Contents
How AI Grammar Checkers Work (Beyond Spellcheck)
Traditional spellcheck compares each word to a dictionary. If the word isn't in the dictionary, it's flagged. That's 1980s technology.
Modern AI grammar checkers work differently. They read the entire sentence (and surrounding sentences) and ask: "Does this make sense as language?" They're not checking against rules — they're evaluating probability. An LLM reads "The cat sat on the mat" and assigns it high probability. It reads "The cat sit on the mat" and assigns it lower probability because the subject-verb agreement pattern is statistically unusual.
This is fundamentally different from rule-based checking:
- Rule-based (old): "Does this violate a grammar rule?" — Misses errors that happen to form real words (their/there/they're).
- AI-based (new): "How likely is a fluent human to have written this?" — Catches contextual errors that rules can't encode.
The tradeoff: AI checkers sometimes flag perfectly correct sentences as errors because the statistical pattern is rare. "The buffalo buffalo Buffalo buffalo buffalo buffalo Buffalo buffalo" is grammatically correct English — an AI checker will flag it as nonsense.
What Grammar Tools Catch — And What They Miss
| Error Type | Detected? | Example |
|---|---|---|
| Spelling (wrong word form) | ✅ Yes | "I want to right a letter" → "write" |
| Subject-verb agreement | ✅ Yes | "The list of items are on the desk" → "is" |
| Their/they're/there confusion | ✅ Yes | "Their going to the store" → "They're" |
| Your/you're confusion | ✅ Yes | "Your welcome" → "You're" |
| Passive voice flagging | ✅ Yes | "The ball was thrown by John" → flagged |
| Comma splices | ✅ Yes | "I went home, I was tired" → flagged |
| Tone/register mismatch | ⚠️ Partial | Using "gonna" in a cover letter → some flag, some don't |
| Factual correctness | ❌ No | "The Eiffel Tower is in London" → not flagged |
| Logical flow between paragraphs | ❌ No | AI can check sentence-level logic but not argument-level coherence |
| Intentional stylistic choices | ❌ No | Fragments for emphasis. Like this. Flagged as errors. |
Free vs Paid Grammar Tools: The Real Differences
The core grammar checking engine is the same in most free and paid tiers. What changes:
- Free: Spelling, basic grammar (subject-verb agreement, punctuation), maybe passive voice detection. Covers 90% of errors.
- Paid: Style suggestions, tone detection, clarity scoring, genre-specific rules (academic, business, creative), plagiarism checking, team features. The extra 10% that matters for professional writers and editors.
For most people, a free grammar checker catches the errors that actually matter — typos, homophone confusion, broken sentences. The paid features are valuable for long-form professional writing where style consistency and plagiarism detection justify the subscription.
How to Use a Grammar Checker Effectively
- Write first, check later. Don't fix errors as they're flagged — it breaks flow. Write the whole draft, then run the grammar check.
- Review every suggestion. Don't "Accept All." Grammar tools have false positives. Read each flagged item and decide.
- Know the rules it enforces. A grammar checker flagging "passive voice" isn't saying it's wrong — it's saying "consider if active voice is stronger here." Sometimes passive voice is correct ("The package was delivered" — we don't know or care who delivered it).
- Combine tools. Grammar checker for mechanics. Readability score for complexity. Human eye for voice. No single tool does everything.
The 10 Most Common Grammar Errors (That Tools Fix Instantly)
- Its / It's — "It's" = it is. "Its" = belonging to it. "The dog wagged its tail."
- There / Their / They're — Location / possession / contraction of "they are."
- Your / You're — Possession / contraction of "you are."
- Affect / Effect — Affect (verb) = influence. Effect (noun) = result.
- Then / Than — Time sequence / comparison.
- Loose / Lose — Not tight / misplace or fail to win.
- Who / Whom — Who = subject. Whom = object. "Who called?" "Whom did you call?"
- Fewer / Less — Fewer for countable things. Less for uncountable. "Fewer apples, less water."
- Me / I — "Between you and me" (not I). Remove other person to check: "The gift was for me" → "The gift was for my wife and me."
- Dangling modifiers — "Walking down the street, the trees were beautiful." The trees weren't walking. "Walking down the street, I saw beautiful trees."
Use the iluv.tools Grammar Checker to catch these and more. Paste your text, it highlights errors with explanations. Free, no account, works in your browser.