Common Grammar Mistakes That Make You Look Unprofessional (And How to Fix Them)

One grammar mistake in a sales email can kill a deal. A few typos in a blog post and readers stop trusting you. A Global Lingo study found that 59% of people would avoid a company whose content had spelling or grammar errors.

The upside? Most grammar mistakes are predictable. Once you learn the usual suspects, they're easy to catch. This guide covers the errors that do the most damage in professional writing and how to stop making them.

Why grammar still matters in the digital age

We have autocorrect and AI writing tools now. So why do grammar mistakes still matter?

• Trust: Grammar errors make you look careless, and carelessness erodes authority
• Professionalism: Clients and employers notice writing quality when making decisions
• Clarity: Bad grammar creates ambiguity or changes meaning entirely
• SEO: Errors aren't a direct ranking factor, but they increase bounce rates and hurt engagement

The 12 most common (and damaging) grammar mistakes

1. Your vs. You're

This might be the single most common error in business writing.

❌ Wrong: "Your going to love this new feature!"
✅ Right: "You're going to love this new feature!"
(You're = You are)

❌ Wrong: "Is this you're final decision?"
✅ Right: "Is this your final decision?"
(Your = possessive, showing ownership)

Quick test: Swap in "you are." If the sentence still works, use "you're." If not, use "your."

2. Its vs. It's

❌ Wrong: "The company announced it's new policy."
✅ Right: "The company announced its new policy."
(Its = possessive)

❌ Wrong: "Its been a great year for sales."
✅ Right: "It's been a great year for sales."
(It's = It is/It has)

Quick test: Try replacing it with "it is" or "it has." If that works, use "it's." Otherwise, go with "its."

3. Their vs. There vs. They're

Their (possessive - belongs to them):
✅ "The team submitted their report on time."

There (location or existence):
✅ "Put the files over there."
✅ "There are three options to consider."

They're (They are):
✅ "They're launching the product next week."

4. Affect vs. Effect

Affect (verb - to influence):
✅ "The new policy will affect our workflow."
✅ "How will this decision affect customers?"

Effect (noun - the result):
✅ "The effect of the campaign was immediate."
✅ "We saw a positive effect on sales."

Exception - Effect as a verb (rare):
✅ "To effect change, we need bold action."
(To effect = to cause/bring about)

Memory trick: Affect = Action (both start with A). Effect = End result (both start with E).

5. Then vs. Than

Then (time/sequence):
✅ "First, write the draft. Then, edit it."
✅ "We launched in 2020, then expanded in 2021."

Than (comparison):
✅ "This tool is faster than the alternative."
✅ "She has more experience than I do."

6. Lose vs. Loose

❌ Wrong: "Don't loose this opportunity!"
✅ Right: "Don't lose this opportunity!"
(Lose = to misplace or fail to win)

❌ Wrong: "The bolt is lose."
✅ Right: "The bolt is loose."
(Loose = not tight)

7. Apostrophe misuse in plurals

This one is everywhere. Apostrophes do not make words plural:

❌ "We sell laptop's, phone's, and tablet's"
✅ "We sell laptops, phones, and tablets"

❌ "The 1990's were great"
✅ "The 1990s were great"

❌ "All employee's must attend"
✅ "All employees must attend"

When to use apostrophes: For possessives (John's book) and contractions (don't, we're). That's it.

8. Comma splices

This happens when you join two complete sentences with just a comma:

❌ "The product launched yesterday, it exceeded our expectations."

Fix option 1 (period):
✅ "The product launched yesterday. It exceeded our expectations."

Fix option 2 (semicolon):
✅ "The product launched yesterday; it exceeded our expectations."

Fix option 3 (conjunction):
✅ "The product launched yesterday, and it exceeded our expectations."

9. Subject-verb agreement

❌ "The team are meeting at 3 PM." (US English)
✅ "The team is meeting at 3 PM."

❌ "Each of the employees have submitted their forms."
✅ "Each of the employees has submitted their forms."
(Subject is "each," not "employees")

❌ "The data shows a clear trend." (plural data)
✅ "The data show a clear trend."
(Or use "dataset is" for singular)

10. Misplaced modifiers

This is when a descriptive phrase ends up too far from the thing it describes:

❌ "We almost increased sales by 50%."
(Did you almost increase them, or increase them by almost 50%?)
✅ "We increased sales by almost 50%."

❌ "Walking down the street, the building looked abandoned."
(The building was walking?)
✅ "Walking down the street, I noticed the building looked abandoned."

11. I vs. Me (and Myself)

❌ "Send the report to John and I."
✅ "Send the report to John and me."
(Test: Remove "John and" → "Send it to me" not "Send it to I")

❌ "Me and Sarah completed the project."
✅ "Sarah and I completed the project."
(Test: "I completed" not "Me completed")

❌ "Please contact myself with questions."
✅ "Please contact me with questions."
(Use "myself" only for reflexive: "I did it myself")

12. Could of / Should of / Would of

People write this because "could've" sounds like "could of" when spoken aloud:

❌ "I should of checked the data."
✅ "I should have checked the data."
(Or "should've")

❌ "We could of won the deal."
✅ "We could have won the deal."

❌ "They would of arrived earlier."
✅ "They would have arrived earlier."

Advanced grammar pitfalls for professional writers

That vs. Which

That (essential information - no commas):
✅ "The report that was submitted yesterday needs revision."
(Specifies WHICH report)

Which (extra information - use commas):
✅ "The report, which was submitted yesterday, needs revision."
(Additional detail about the report)

Memory trick: "That" is essential TO the sentence. "Which" adds WHICH extra detail.

Who vs. Whom

Who (subject - doing the action):
✅ "Who wrote this email?"
(He wrote it = "who")

Whom (object - receiving the action):
✅ "To whom should I address this letter?"
(Address it to him = "whom")

Modern usage: "Who" is increasingly accepted in both cases for informal writing.

Fewer vs. Less

Fewer (countable items):
✅ "We have fewer customers this month."
✅ "Fewer than 10 people attended."

Less (non-countable amounts):
✅ "We spent less money on ads."
✅ "There's less traffic today."

Punctuation mistakes that change meaning

Why commas matter

One misplaced comma can completely change what you're saying:

"Let's eat, Grandma!" (Calling Grandma to dinner)
vs.
"Let's eat Grandma!" (Planning to eat Grandma!)

"The panda eats, shoots and leaves." (Three actions)
vs.
"The panda eats shoots and leaves." (Diet description)

Semicolon usage

Use semicolons to:

1. Join related independent clauses:
✅ "The proposal was approved; implementation begins Monday."

2. Separate complex list items:
✅ "Attendees included John Smith, CEO; Sarah Jones, CTO; and Mike Brown, CFO."

Don't use semicolons to:
❌ Introduce a list: "We need; pens, paper, and folders."
(Use a colon instead)

How to catch grammar mistakes before publishing

1. Read it out loud: Your ear will catch things your eyes skip over
2. Use more than one tool: Grammarly, Hemingway Editor, ProWritingAid all catch different things
3. Print it out: Errors are easier to spot on paper than on screen
4. Know your weak spots: Keep a list of mistakes you make often
5. Use our text tools: Remove extra spaces, check character counts, find and replace errors
6. Get a second reader: Have someone else look over anything important
7. Step away first: Take a break before editing so you come back with fresh eyes

Free tools to improve your writing

Clean grammar gives you an edge

When everyone can publish, good grammar is what separates the professionals from the amateurs. It shows you pay attention, you care about your work, and you respect the person reading it.

You don't need to memorize every rule. Bookmark this page, run your writing through a grammar checker, and pay attention to the mistakes you keep making. After a while, getting it right becomes second nature.

Want to clean up your text formatting? Our Remove Extra Spaces and Find and Replace tools can help you tidy things up fast.