5 Grammar Mistakes a Professional Engine Catches That You Might Miss
A practical guide to common English grammar errors — and how Harper, an open-source grammar engine, detects them automatically.
Good writing is not just about vocabulary and style — small mechanical errors can undermine even a polished piece in seconds. Most of these mistakes follow predictable patterns, which is exactly what a rule-based grammar engine excels at catching.
Our Grammar Checker is powered by Harper, an open-source grammar engine written in Rust. Here are five of the most common errors it reliably detects.
1. Repeated consecutive words
The most embarrassing typo is writing the same word twice in a row — “the the”, “is is”, “and and”. It happens when editing mid-sentence and forgetting to remove the duplicate. Most spell-checkers miss it entirely because both words are spelled correctly.
Example:
- ✗ “She went to the the store.”
- ✓ “She went to the store.”
2. Incorrect article usage: a vs. an
Use an before words with a vowel sound, a before words with a consonant sound. The key is pronunciation, not spelling.
- ✓ “an umbrella” — vowel sound
- ✓ “a unicorn” — sounds like “you” (consonant sound)
- ✗ “a apple” — should be “an apple”
- ✗ “an user” — should be “a user”
This trips up even native speakers with words like “hour” (silent h → “an hour”) and “historical” (aspirated h → “a historical”).
3. Capitalization errors
Every sentence must start with a capital letter. Harper also catches cases where the pronoun I is written in lowercase — a common habit in informal messages and chat.
Examples:
- ✗ “yesterday i went to the market.” → “Yesterday I went to the market.”
- ✗ “i think it’s a great idea.” → “I think it’s a great idea.”
4. Spelling mistakes
Harper includes a built-in English dictionary and flags words that do not match any known spelling. Unlike simple pattern matching, it suggests the most likely correction.
Examples:
- ✗ “recieve” → “receive”
- ✗ “occured” → “occurred”
- ✗ “definately” → “definitely”
5. Punctuation spacing
After a comma or semicolon, there must be a space before the next word. Missing spaces often appear when copying text from different sources or typing quickly on mobile.
Examples:
- ✗ “Hello,world” → “Hello, world”
- ✗ “first;second” → “first; second”
How to use the Grammar Checker
Paste any English text into our free Grammar Checker and Harper analyses it automatically after a short pause. Each issue shows:
- A category label (Grammar, Style, Punctuation)
- A clear description of the problem
- A suggested fix
- A line and column number so you can find the issue instantly
Your text is sent over HTTPS to belun.app’s own API server and processed by Harper. Nothing is stored or shared.
Beyond the basics
For issues no automated tool catches — nuanced subject-verb agreement, comma splices, dangling modifiers — read your text aloud. Sentences that are hard to say are usually hard to read. If something sounds awkward, it likely needs restructuring rather than a mechanical fix.
Try the Grammar Checker — paste any English text and get instant professional feedback, no signup required.