In a previous post, I wrote that using a comma after an introductory dependent clause is standard practice, no thinking required. To refresh your memory, here’s a sentence that begins with an introductory dependent clause:
If you’d like to go to Europe next summer, let’s get together and make some plans!
Things aren’t quite so simple when the dependent clause follows an opening independent clause; in this case, the determining factor for the use of a comma is whether the dependent clause is restrictive or nonrestrictive.
According to the Chicago Manual of Style, a restrictive dependent clause is one that is “essential to the meaning of the main clause.” It shouldn’t be preceded by a comma. Following are two examples:
I’ll attend the conference if you will. [The restrictive dependent clause is “if you will.”]
Andrew was thrilled when he heard about her new book. [The restrictive dependent clause is “when he heard about her new book.”]
On the other hand, says Chicago, a nonrestrictive dependent clause “is merely supplementary” and should be preceded by a comma:
I have already made my reservation, if that interests you. [The nonrestrictive dependent clause is “if that interests you.”]
People whose thoughts are mostly positive are happier than people whose thoughts are mostly negative, if you want my opinion. [The nonrestrictive dependent clause is “if you want my opinion.”]
But what if you really can’t tell if a dependent clause is restrictive or nonrestrictive? In that case, Chicago advises using the comma.
Tara’s Writing Studio