The most common introductory word groups are clauses and phrases functioning as adverbs. Such word groups usually tell when, where, how, why, or under what conditions the main action of the sentence occurred. (See 48a, 48b, and 48e.)

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.

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 participial phrases that function as adjectives, describing the noun or pronoun immediately following them. 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 48b.)

note: Other introductory word groups include transitional expressions and absolute phrases (see 32f).