You should limit the number of times that you interrupt the flow of a sentence by placing modifying words between the subject and its verb. When you do introduce such appositives, participial phrases, or adjective phrases or clauses, you must determine whether the modifiers are restrictive or nonrestrictive. Essentially, restrictive modifiers add information that is essential to the meaning of the sentence, whereas nonrestrictive modifiers add information that is not essential. The best way to determine whether a modifier is restrictive or nonrestrictive is to see if taking it out changes the meaning of the sentence.
Restrictive: Lawyers who work for McGullity, Anderson, and Swenson need to take a course in copyediting.
In this case the relative clause is essential to the meaning of the sentence. If you embedded the clause in commas, then the meaning would change, suggesting that all lawyers need a course in copyediting.
Restrictive: The lawyer who has worked on this case for three years thinks that we have no chance of winning.
In this case the relative clause is essential to the meaning of the sentence. In other words, the sentence refers to only the lawyer who has worked on this case. The discussion is restricted to her.
Nonrestrictive: The lawyers, who have an office downtown, think that we have no chance of winning.
Because the location of the lawyer's office is superfluous to the gist of the sentence, it should be set off by commas.
Other comma resources:
- Use a Comma After Conjunctive Adverbs and Transitional Phrases at the Beginnings of Sentences
- Use Commas After Introductory Subordinate Clauses
- Use Commas Before Nonrestrictive Adverbial Phrases or Clauses at the Ends of Sentences
- Use Commas to Join Two or More Independent Clauses
- Use Commas to Separate Adjacent Parallel Elements