comma with introductory elements

The comma after a word, phrase, or clause that precedes the main clause.

Place a comma after an introductory element unless the element is short and the sentence is clear without one. An introductory element is a word, phrase, or clause that stands before the subject of the main clause and prepares the reader for what follows: When the alarm sounded, the guests filed outside.

An introductory subordinate clause takes a comma regardless of length. The clause contains its own subject and verb, yet a subordinating conjunction (if, when, because, although, and others) prevents it from standing as a complete sentence. The comma follows the whole clause, not the subordinating conjunction that opens it: If the bus is late, text me. Although the roads were icy, the bus ran on time.

The need for a comma after an introductory phrase depends on the phrase type and its length. A participial phrase, an infinitive phrase, or an absolute phrase requires a comma: Tired from the drive, Derek went straight to bed. To reach the summit by noon, the hikers left at dawn. Her shift over, Karen drove home. A long prepositional phrase or a series of prepositional phrases takes a comma: After the long meeting about the budget, the staff went to lunch. The comma after a short prepositional phrase is optional when the sentence remains clear: In March, the lease expires.

The need for a comma after an introductory word depends on the word’s function. A transitional expression that connects the sentence to the preceding one, or a sentence adverb that comments on the statement as a whole, takes a comma: Meanwhile, the rain had stopped. Fortunately, no one was hurt. An adverb that modifies the verb may stand without a comma: Slowly the fog lifted. A coordinating conjunction (and, but, or) that opens a sentence takes no comma: But the store had already closed.

Even a short introductory element takes a comma when its omission would produce a misreading. Without the comma, Below the river glittered in the sun allows the reader to treat below the river as a single phrase. The comma in Below, the river glittered in the sun marks below as the introductory element and the river as the subject.

Some opening phrases are not introductory elements and therefore take no comma. A phrase that serves as the subject of the sentence is never separated from its verb by a comma: Sorting the mail took most of the morning. An inverted construction in which the verb precedes the subject likewise takes no comma after the opening phrase: Beside the door hung a brass bell.

Examples

Use Because the elevator was broken, we took the stairs. (Because the elevator was broken is an introductory subordinate clause, so a comma follows it.)

Use Holding the ladder steady, Juan reached for the gutter. (Holding the ladder steady is an introductory participial phrase, so a comma follows it.)

Use On Tuesday the clinic opens late. (On Tuesday is a short prepositional phrase, and the sentence is clear without a comma.)

Avoid After the bus left the station emptied. (After the bus left is an introductory subordinate clause, so a comma is required after left. Write After the bus left, the station emptied.)

Avoid Painting the fence, took all weekend. (The comma after fence is incorrect. Painting the fence is the subject of took, and a comma never separates a subject from its verb. Write Painting the fence took all weekend.)

Avoid Beside the door, hung a brass bell. (The comma after door is incorrect. This is an inverted construction in which the verb precedes the subject, so no comma follows the opening phrase. Write Beside the door hung a brass bell.)