The 20 Most Common Student Errors
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 
Connors, Robert J., and Andrea Lunsford. "Teachers' Rhetorical Comments on Student Papers." CCC 44 (1993): 200-23.