In my ten years of composition instruction, I have developed a set of pet peeves associated with the body of student writing I have read. Any of my students reading this should keep in mind that I do not direct this at any particular student — this list is a synthesis of common writing errors that I often find in student papers at every grade level 6-12 and every academic level, including Honors or AP.
• Run-ons, comma splices, and fragments. Subject+verb+complete thought=sentence. Commas cannot join independent clauses. Independent clauses cannot simply be mashed together either. Let me introduce you to the semicolon. He is your friend.
No question about it: the semicolon is my friend.