In 1993, Robert Connors and Andrea Lunsford published their results after studying the most common errors marked on 21,000 student essays. Generally, teachers consider syntactic or sentence-level errors (sentence fragments, fused sentences, comma splices) more serious than word-level errors (spelling, verb forms, agreement). The list below is ordered with most frequent errors first.
 

1. Missing comma after an introductory element
2. Vague pronoun reference
3. Missing comma in a compound sentence
4. Wrong word
5. Missing comma(s) after a nonrestrictive element
6. Wrong or missing verb ending
7. Wrong or missing preposition
8. Comma splice
9. 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 agreement between subject and verb
15. Missing comma in a series
16. Lack of agreement between pronoun and antecedent
17. Unnecessary comma(s) with a restrictive element
18. Fused sentence
19. Dangling or misplaced modifier
20. Its/it's confusion