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.