Here is a list of the 20 most-common writing errors. By clicking on an error, you can view an example and description of it. You can also download them all in this PDF. (If you do not already have the Adobe PDF Reader on your computer, you can download it for free here.)

01. Missing comma after an introductory element
02. Vague pronoun reference
03. Missing comma in a compound sentence
04. Wrong word
05. Missing commma(s) with a nonrestrictive element
06. Wrong or missing verb ending
07. Wrong or missing preposition
08. Comma splice
09. Missing or misplaced possessive apostrophe
10. Unnecessary shift in tense
11. Unnecessary shift in pronoun
12. Sentence fragment
13. Wrong tense or verb form
14. Lack of subject-verb agreement (continued)
15. Missing comma in a series
16. Lack of agreement between pronoun and antecedent (continued)
17. Unnecessary comma(s) with a restrictive element
18. Fused sentence
19. Misplaced or dangling modifier
20. Its/It’s confusion

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Professor Paul Brians’s website is such a vast source of information about errors in English usage that it gave birth to a book: