Use a comma to separate a group of prepositional phrases of more than four words when the phrases come at the beginning of a sentence.
Do not use a comma between separate phrases unless they are in a series.
A comma may also set off a single prepositional phrase at the beginning to make the sentence clear. A comma is recommended after any introductory prepositional phrase of more than four words.
Correct: Under the kitchen table the dog cowered.
(Single short, clear phrase. No comma needed.)
Correct: Under the spreading chestnut tree, the village smithy stands.
(Comma optional, but helpful due to length of phrase)
Correct: Under the pile of clothes, we found his wallet.
(Two prepositional phrases, not in a series)
Incorrect: On the sand, of the beach, by the inlet, we relaxed in the sun.
(Do not separate the phrases since they are not in a series.)
Correct: On the sand of the beach by the inlet, we relaxed in the sun.
Correct: Over hill, over dale, we hit the dusty trail.
(The two phrases are in series here. We could say "Over hill and over dale.")
Introductory Participial and Infinitive Phrases
Use a comma to separate introductory participial phrases and infinitive phrases used as modifiers.
Correct: Looking for help, the man fell on his knees to beg.
Correct: To raise enough money in time, Mary had to issue stock in her business.
(The infinitive phrase is used as a modifier)
Incorrect: To ski, is exhilarating.
(The infinitive is used as a noun, not a modifier.)
Correct: To ski is exhilarating.