When words which "set the scene" for the main part of the sentence appear at the front of the sentence, it is usual to offset them with a comma. For example:|
When the introduction is just one word (e.g., Yesterday, Here, Now), it is common practice to omit the comma.
Using a Comma after an Introductory Adverbial Clause or PhraseIt is common for a sentence to start with an introduction. An introduction can be anything from just one word to a long clause.
An introduction is used to state a time, a place, a condition, a frequency, or a fact before the main part of the sentence.
(These "introductions" vary hugely. They are known as dependent clauses because they cannot stand alone as complete ideas. If an "introduction" contains its own subject and verb, it will be an adverbial clause, otherwise it will be an adverbial phrase. The main part of the sentence (i.e., the clause after the "introduction") is called an independent clause.)
Read more about clauses and phrases.
Examples of Introductory Adverbial Clauses and PhrasesIn these examples, the introductory adverbial clauses and phrases are shaded:
ONLY USE A COMMA FOR AN INTRODUCTION
Words that "set the scene" do not always start the sentence; they could appear at the back or in the middle. A comma is only expected when these words appear at the front of the sentence. This is very handy to know. Look the examples below:
A LOT OF LENIENCY ON THIS RULING
Regardless of where the scene-setting words appear (start, middle or end), there is some leniency on whether to use a comma or not. The primary purpose of a comma (or commas if it appears in the middle) is to aid your reader. Above all else, making your text easy to read should determine whether you use a comma or not.
Note: When the introduction is a participle phrase describing the subject of the sentence, you must separate the participle phrase and the subject with a comma. For example:
Most of the scene-setting words are classified as adverbs. When they are made up of more than one word, they are called adverbial phrases or clauses. When they're just one word, it is a common practice to drop the comma. For example: