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.

Cheers,

Tara Treasurefield
Tara’s Writing Studio