Quick Takeaways
Grammar mistakes quietly undermine your credibility, and the most common ones are easy to fix once you can spot them. This guide covers 10, from the its and it's mix-up to passive voice and run-on sentences. A few you can catch with a free tool, and the rest you catch by slowing down to proofread.
Grammar mistakes are the fastest way to make sharp writing look careless, and readers notice more than you'd think. A single "your" where you meant "you're" can pull a reader out of an otherwise strong email or post. The ideas might be excellent, but the slip is what sticks in memory.
The good news is that a small handful of errors account for most of the damage. Learn to spot these 10 and you'll clear the mistakes that make professional writing read as amateur.
Some of them a free tool can flag for you, and the rest come down to a careful read. Let's go through all 10, grouped so they're easier to remember and easier to catch next time you write.
Table of Contents
What Makes a Grammar Mistake Look Unprofessional
Grammar Mistakes With Apostrophes and Confused Words
Sentence-Structure Errors Readers Notice
Wordy Habits That Weaken Your Writing
Which Grammar Mistakes UltraWordCounter Can Catch
Frequently Asked Questions
Conclusion
What Makes a Grammar Mistake Look Unprofessional
A grammar mistake reads as unprofessional when it signals that nobody checked the work. Readers rarely know the technical rule you broke, but they feel the friction, and that friction reads as carelessness. In a cover letter or client email, that impression can quietly cost you the opportunity.
Not every error carries the same weight. A single missed comma is minor, while "your" for "you're" in a headline can undercut your entire message. The 10 mistakes below are the ones people notice most, so they're the ones worth fixing first.
Think of them as the difference between a draft and a finished piece. Most take seconds to correct once you know the pattern, and spotting one usually trains your eye to catch the rest. That small habit is what separates writing that reads as polished from writing that reads as rushed.
The 10 grammar mistakes covered in this guide, at a glance.
Grammar Mistakes With Apostrophes and Confused Words
These four trip up strong writers because spellcheck usually misses them. Each word is spelled correctly, so the tool stays quiet even though you picked the wrong one for the sentence.
1. Its vs. it's. The form "it's" is short for "it is," while "its" shows possession. If you can swap in the two-word version and the sentence still works, use the apostrophe; otherwise, drop it.
2. Your vs. you're. The contraction "you're" means "you are," while "your" shows possession. Writing "your welcome" accidentally says something you don't mean, so mentally expand the word to check it.
3. Then vs. than. Use "than" for comparisons, as in "faster than before," and "then" for time or sequence. Swapping the two is a small slip that a careful reader spots instantly.
4. Affect vs. effect. Usually "affect" is a verb and "effect" is a noun: rain affects your mood, the effect a bad day. When unsure, test whether you need an action or a thing.
A quick test for the four confused-word grammar mistakes.
Sentence-Structure Errors Readers Notice
The next three break the way a sentence holds together. They're harder to see because the words are all correct, so the problem hides in how those words are joined.
5. Comma splices. Joining two complete sentences with only a comma creates a splice. Fix it with a period, a semicolon, or a comma plus a joining word, a rule laid out clearly in Purdue OWL's guide to commas.
6. Run-on sentences. When two or more complete thoughts collide with no punctuation between them, readers lose the thread halfway through. Break the run-on into shorter sentences so each idea lands on its own.
7. Dangling modifiers. "Walking into the room, the lights were off" accidentally says the lights were walking. Put the person doing the action right after the opening phrase, so the sentence describes who you actually mean.
Wordy Habits That Weaken Your Writing
The last three don't break rules so much as drain your writing's energy. Fix them and the very same ideas read sharper and more confident.
8. Overusing passive voice. "Mistakes were made" hides who acted, and overuse makes writing feel flat, as Purdue OWL's guide explains. "We made a mistake" is shorter and more honest.
9. Wordiness. Phrases like "due to the fact that" and "in order to" pad a sentence without adding meaning. Cut them down to "because" and "to," and your point lands faster.
10. Subject-verb agreement. A singular subject needs a singular verb: write the team is, not the team are. Long sentences hide these slips, so match the verb to its real subject, not the nearest noun.
Which Grammar Mistakes UltraWordCounter Can Catch
Here's the honest part. UltraWordCounter isn't a full grammar checker, so it won't fix your apostrophes or catch "then" written for "than."
What it does do is flag the clarity problems on this list, the ones that quietly drag your writing down. Those are the errors that make a page feel sloppy even when every word is spelled right.
Paste your draft into UltraWordCounter and it flags passive voice and overlong sentences instantly. Everything runs in your browser, with nothing uploaded and nothing saved, so your draft stays on your device.
That covers mistakes 6 and 8 directly, and it helps you spot the wordiness in mistake 9. For the clarity pass, the free reading grade level and passive voice checker flags long sentences and passive constructions. You fix the flagged spots, recheck, and watch the writing tighten.
For the rest, the homophones and apostrophes, the fix is old-fashioned. Read your work aloud before you send it, because your ear catches "your" for "you're" faster than your eyes do.
Everything runs free up to 800 words per session, which covers most emails and posts. Questions about how the passive voice flag works are answered on the UltraWordCounter FAQ page.
If you edit long documents all day, you'll want unlimited checks. The Pro plan removes the 800-word cap for $19 a month, or $12 if you pay annually ($144/year), with a 7-day free trial. If your checks stay short, stay free; that's fine with us.
What the clarity check flags, and which mistakes you proofread yourself.
Frequently Asked Questions
What are the most common grammar mistakes in professional writing?
The ones readers notice most are confused words like its and it's or your and you're, comma splices, run-on sentences, and overused passive voice. None of them are complex rules, yet each signals rushed work. Catching this short list clears most of the errors that make writing look careless.
Does UltraWordCounter check my grammar?
Not in full, since UltraWordCounter isn't a grammar checker and won't fix apostrophes or homophones. It does flag passive voice, overlong sentences, and wordiness, which are the clarity mistakes on this list. For spelling and word-choice slips, proofread carefully or read your draft aloud before sending.
How do I stop confusing its and it's?
Use one quick test whenever you're unsure. Replace the word with "it is" and reread the sentence: if it still makes sense, you want "it's" with the apostrophe. When it sounds wrong, you want "its," the possessive form, with no apostrophe at all.
Are grammar mistakes really a big deal at work?
They can be. One obvious error in a cover letter or client email suggests you didn't proofread, which reads as carelessness even when your ideas are strong. In competitive situations, clean writing signals attention to detail and quietly builds a reader's trust in you.
What's the fastest way to catch grammar mistakes before sending?
Read your writing aloud, then run it through a tool for clarity issues. Your ear catches wrong words and clunky sentences that your eyes skip right over, and a free checker flags passive voice and overlong sentences. Together, the two habits catch far more than a silent reread.
Conclusion
Grammar mistakes rarely sink you on their own, but they chip away at how seriously readers take your work. Master these 10, from the its and it's mix-up to passive voice and run-on sentences, and your writing instantly reads more polished and more professional.
Handle it in two passes. Read your draft aloud to catch the confused words, then paste it into UltraWordCounter to flag passive voice and sentences that run too long. Try it free and send your next email knowing it reads as sharp as your thinking.
Written by the UltraWordCounter editorial team: editors who proofread real emails, posts, and client work daily. We run each draft through a clarity check first, because we make these same mistakes too.