Essential and Nonessential Dependent Clauses
A dependent clause is a clause that cannot stand on its own as a sentence. Dependent clauses are often introduced by words such as the following: because, since, until, after, before, while, which, that, who, so, for.
If a dependent clause comes at the beginning of a sentence, set it off with a comma. For example, “Until I give the word, the color guard members should stay right where they are.” “Until I give the word” is a dependent clause.
If a dependent clause comes in the middle of a sentence or at the end of a sentence, the writer has to make a decision about how to punctuate it. If the dependent clause is essential to the meaning of the sentence, do not set it off with commas. If the dependent clause is extra information, do set the dependent clause off with commas. For example, in the sentence “Abraham Lincoln, who was president during the Civil War, gave the Gettysburg Address.” the dependent clause is “who was president during the civil war.” If you think your reader knows who Abraham Lincoln was, then the dependent clause is extra, nonessential information. You hide that extra information behind commas.
Now think about the sentence “The man who was president during the Civil War gave the Gettysburg Address.” In this sentence, the dependent clause identifies which “man” we mean. In this case, the dependent clause is essential information, so you don’t hide it behind a comma.
I’ve just explained the basic principle for punctuating dependent clauses. Applying the principle in real writing calls for some judgment on the part of the writer. I hope the exercises will help you understand the principle more fully.