Adverbial Phrases

We often use a comma to separate multiword adverbial phrases or clauses from the rest of the sentence when the phrase or clause comes first. In these next examples you’ll recognize a dependent clause, a prepositional phrase, a participial phrase, and an infinitive phrase.

• When the clock strikes twelve, my dog howls. 
• Over the course of a very long night, the surgeon operated on three children. 
• Wanting to earn a place on the podium, the runner accepted the steroids. 
• To make her decision, Mary studied each option for hours. 

The use of commas after prepositional phrases isn’t always straightforward. We use a comma after some prepositional phrases at the beginning of a sentence but not after others. So when do you use a comma and when don’t you?

1. Use a comma after prepositional phrases of five or more words:
On the counter sat a bundle of old letters. (fewer than five words)
From the elevator’s control panel, red paint dripped like blood. (five or more words)

2. Use a comma to eliminate confusion:

In the contest entries were arranged alphabetically. (confusing)

In the contest, entries were arranged alphabetically. (Without a comma after contest, readers might assume that contest entries was a unit, with contest modifying entries.)

3. Use a comma when a series of prepositional phrases starts the sentence:

During drought and famine, in good times and bad, I’ll be there for you. (A series of two prepositional phrases.)

(from The Editor's Blog A Tale of Adverbs and the Comma)