Among its four uses, the structure comma signals that subordinate information precedes the independent clause of a sentence.

Use the Comma with Introductory Dependent Clauses and Phrases

The normal grammatical order of English puts independent clauses before dependent clauses at the start of sentences:

They missed the bus because they went to the store.

In this example, the independent clause “They missed the bus” occurs before the dependent clause “because they went to the store.” This follows normal grammatical order for English sentences, so no structure comma is needed. But when a dependent clause occurs before an independent clause at the start of a sentence, a comma is required:

Because they went to the store,↑they missed the bus.

A comma is also necessary when a verbal or prepositional phrase appears at the start of a sentence:

Despite running half a city block,↑they missed the bus.

For the second time that week,↑they had to take a cab.

Stacked introductory phrases and dependent clauses also need commas:

The next day,↑to avoid another argument,↑they arrived at the bus stop early.

Some style guides allow for short introductory phrases to precede the independent clause of a sentence without a comma if the resulting sentence is short and its meaning clear. But as with other exceptional rules like this, readers can have different opinions about what is “short” and what is “clear.” For this reason, consider using the comma with all introductory dependent clauses and phrases, unless you have a good reason for doing otherwise.

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