Grammar Checkers: What They Catch and What They Miss

Grammarly charges $12/month and sends your text to their servers. A good browser-based grammar checker runs 200+ rules locally — no API, no subscription, no privacy concerns. But it won't catch everything. Here's what grammar checkers actually do, where they fall short, and how to use them effectively.

What a Rule-Based Grammar Checker Detects

Unlike AI-powered tools that rewrite your text, a rule-based checker pattern-matches against known error categories. It's deterministic — the same input always produces the same output. The categories typically include:

• Subject-verb agreement: "The list of items were on the table" → the subject is "list" (singular), not "items."
• Article errors: "I bought a apple" → should be "an apple."
• Double negatives: "I don't have no money" → should be "any money."
• Commonly confused words: their/there/they're, your/you're, its/it's, affect/effect, then/than.
• Missing or extra apostrophes: "The dogs bowl" → "The dog's bowl." "The Smith's house" → "The Smiths' house" (plural possessive).
• Capitalization: Days of the week, months, proper nouns, sentence starts.
• Repeated words: "the the" — these slip through spell check because each word is valid on its own.
• Hyphenation: "well known" before a noun should be "well-known" ("a well-known author" vs. "the author is well known").

What Grammar Checkers Miss

Rule-based checkers are only as good as their rules. They consistently miss:

• Passive voice overuse: Some checkers flag passive voice, but most can't tell when it's appropriate. "The ball was hit by John" is passive. But "The window was broken during the storm" is also passive — and it's fine, because the actor is irrelevant.
• Word choice and tone: "Utilize" isn't wrong, but "use" is better. "Very unique" is technically redundant. A rule-based checker won't catch these style issues.
• Context-dependent errors: "I'll see you their" — a spell checker won't flag it because "their" is a valid word. A grammar checker should catch it if it has a rule for there/their/they're, but the rule needs context to work.
• Sentence-level clarity: A 40-word sentence with three subordinate clauses isn't technically wrong, but it's hard to read. Checkers don't measure readability well.
• Hedging and weak language: "I think maybe we should possibly consider..." — three hedging words in a row. A human editor strikes two of them.
• Consistency: If you write "color" in paragraph 1 and "colour" in paragraph 5, most checkers won't notice — unless they have a specific consistency rule.

Browser-Based vs. Cloud-Based: The Privacy Question

Most grammar checkers (Grammarly, ProWritingAid, LanguageTool's API) send your text to a server for processing. This means:

• Your text passes through servers you don't control.
• Confidential documents — legal drafts, medical notes, business plans — leave your device.
• The company may store or analyze your text to improve their models.

A browser-based checker runs all its rules in JavaScript, locally. Your text never leaves your device. This makes it suitable for confidential documents. The tradeoff: it's limited to pattern-matching rules, not AI-powered rewrites.

How to Use a Grammar Checker Effectively

1. Run it after your first draft, not during. Writing and editing are different cognitive modes. Let the words flow first; fix them later.
2. Don't accept every suggestion. Some rules flag valid constructions. "Starting a sentence with a conjunction" isn't wrong — it's a style choice. Use your judgment.
3. Learn from the patterns. If the checker keeps flagging the same error type (say, comma splices), take note. You're learning your weak spots.
4. Don't rely on it for final polish. Read the text aloud after running the checker. Your ear catches awkward phrasing that rules miss.
5. Use it for second-language writing. If English isn't your first language, a grammar checker catches article errors and preposition mistakes that are hard to self-edit.

The Bottom Line

1. Rule-based checkers catch mechanical errors (subject-verb agreement, articles, confused words). They miss style and context.
2. Use a browser-based checker for confidential documents — your text stays on your device.
3. Don't blindly accept suggestions. Some flag valid constructions.
4. Read aloud after checking. Your ear catches what rules miss.
5. Use it to learn your patterns, not just to fix individual errors.

Disclaimer: This guide is for informational purposes only. No grammar checker replaces careful proofreading for important documents.