Introductory elements consist of clauses, phrases and words that appear before the main clause of the sentence. Essentially, they prepare your readers for what the sentence is really about, or the meat of the sentence.
Introductory clauses and phrases that require an offsetting comma include prepositional phrases and clauses, infinitive phrases, participle phrases, dependent/subordinate clauses, nonessential appositive phrases and absolute phrases. The element is a clause if there is a subject and a verb, and it is a phrase if both of these parts of speech are not present. What comes after the introductory element is always an independent clause, or complete sentence, on its own, and the introductory element gives meaning to it. Consider the following examples where omitting the comma is grammatically incorrect:
Prepositional: The phrase/clause starts with a preposition and is more than three words.
Example 1: For the cat that jumped down from the tree, it was a long way to the ground.
Example 1 is a prepositional introductory clause because it has a subject (cat) and a verb (jumped).
Example 2: In the heat of the moment, many people make rash decisions.
Example 2 contains a prepositional introductory phrase because there is a subject (moment) but no verb.
Infinitive: The phrase starts with an infinitive, which is the word “to” followed by a simple verb.
Example 3: To dance all night, she must have maintained tip-top physical conditioning.
Participle: The phrase starts with a present or past participle of a verb. Present participles for regular verbs end with “ing,” and past participles for regular verbs ending with “ed.” Sometimes both forms deviate from this structure if irregular verbs are used, such as built/built, chose/chosen or sang/sung. Despite the type of verb, these phrases modify the noun that immediately follows them.
Example 4: Splashing through the puddles, the girls looked like they were having a blast.
Example 5: Built in the 1920s, the house had a stronger foundation than originally thought.
Dependent/Subordinate: The clause starts with either a subordinate conjunction, such as after, while or if, or a relative pronoun, such as which, whichever or who. The clauses cannot stand on their own because they are incomplete thoughts.
Example 6: If you are not ready to commit, you are unlikely to experience success while trying to lose weight (“if” is a subordinate conjunction).
Example 7: Whether you like to swim in pools or fresh water, you can enjoy excellent results from working swimming into your workout plan (“whether” is a relative pronoun).
Nonessential appositive: These phrases are modifying ones that immediately precede the noun or pronoun they modify. They provide additional meaning or information to the sentence as a whole.
Example 8: An excellent teacher herself when it comes to cooking, my aunt is taking a culinary class.
Absolute: These phrases have a noun or pronoun, a participle (a form of a verb) and any relevant modifiers. They do not modify one word or subject; instead, they modify the entire sentence by providing additional information.
Example 9: Their faces glowing with excitement, the members of the winning team celebrated long after they had won the championship (the introductory phrase modifies the entire clause that follows it).
When any of these types of clauses are at the end of the sentence, the comma is unnecessary unless the clause/phrase contrasts the main clause dramatically. For example:
Incorrect: You can save money on groceries, whether you use coupons or store-specific savings cards.
Correct: You can do well with freelance writing when you have a strong grasp of grammar rules.
Correct: She was crying earlier in the day, even though she had one of the best days of her life (this is an extreme contrast, so the comma is necessary).
Introductory words are often used to help sentences flow well from one to the next. In addition, some are common expressions (of course, on the other hand) or unforceful interjections (yes, well). These words do not have any grammatical connection to the remainder of the sentence. Common ones include the following: however, still, meanwhile, furthermore and others.