Sometimes it's laziness or impatience; sometimes they're genuinely confused.
Using data from millions of its subscribers, Microsoft recently rounded up a list of the top 10 grammar mistakes in the English language. The data comes from people who use Microsoft Word and/or Outlook, both of which come with a tool called Editor.
Editor highlights spelling and grammar errors and makes suggestions to help improve your writing.
It can make a bad impression if you have a document riddled with mistakes, says Nicole Michel, a linguist who is also a project manager at Microsoft.
She told Business Insider: "If you send a CV to a potential employer and it's full of grammar or spelling mistakes, it's going to show carelessness and lack of attention to detail, and also it shows that you're not really putting emphasis and importance on the task."
Below, we've listed the 10 most common grammar errors, along with examples, according to Microsoft. There's a good chance you've made at least one.
1. Leaving too many white spaces between words
Example: To the left.
Correct: To the left.
2. Missing a comma
Example: If the weather remains the same we'll leave early.
Correct: If the weather remains the same, we'll leave early.
3. Missing a comma after an introductory phrase
Example: First of all we must make sure that the power is off.
Correct: First of all, we must make sure that the power is off.
4. Missing a hyphen
Example: My 3 year old son
Correct: My 3-year-old son
5. Incorrect subject-verb agreement
Example: The cats eats.
Correct: The cats eat.
6. Incorrect capitalization
Example: It's cold, But we are going out.
Correct: It's cold, but we are going out.
7. Mixing up possessive and plural forms
Example: My sisters car is old.
Correct: My sister's car is old.
8. Incorrect agreement with noun phrases
Example: I would like to buy this apples.
Correct: I would like to buy these apples.
9. Commonly confused words
Example: After all that running, I am out of breathe.
Correct: After all that running, I am out of breath.
Read Microsoft's list of most commonly confused words.
10. Incorrect verb form after auxiliary
Example: They had ate when we arrived.
Correct: They had eaten when we arrived.