30b Use a comma after an introductory clause or phrase.

A comma tells readers that the introductory clause or phrase has come to a close and that the main part of the sentence is about to begin. The most common introductory clauses and phrases function as adverbs. Such word groups usually tell when, where, how, why, or under what conditions the main action of the sentence occurred. (See 42a, 42b, and 42e.)

The example reads,

When Irwin was ready to iron (insert) comma (end inserts) his cat tripped on the cord.

Without the comma, readers may think that Irwin is ironing his cat. The comma signals that ‘his cat’ is the subject of a new clause, not part of the introductory one.

EXCEPTION: The comma may be omitted after a short adverb clause or phrase if there is no danger of misreading: In no time we were at 2,800 feet.

Sentences also frequently begin with a participial phrase that functions as an adjective, describing the noun or pronoun immediately following it. The comma tells readers that they are about to learn the identity of the person or thing described; therefore, the comma is usually required even when the phrase is short. (See 42b.)

NOTE: Other introductory word groups include transitional expressions and absolute phrases (see 30f).