Research for this book shows that five of the most common errors in college writing involve commas.
Check that a comma separates an introductory word, phrase, or clause from the main part of the sentence. (35a)
Look at every sentence that contains a coordinating conjunction (and, but, for, nor, or, so, or yet). If the groups of words before and after this conjunction both function as complete sentences, use a comma before the conjunction. (35b)
Look at each adjective clause beginning with which, who, whom, whose, when, or where and at each phrase and appositive. If the rest of the sentence would have a different meaning without the clause, phrase, or appositive, do not set off the element with commas. (35c)
Make sure that adjective clauses beginning with that are not set off with commas. Do not use commas between subjects and verbs, verbs and objects or complements, or prepositions and objects; to separate parts of compound constructions other than compound sentences; to set off restrictive clauses; or before the first or after the last item in a series. (35i)
Do not use a comma alone to separate sentences. (See Chapter 28.)