A prepositional phrase begins with a preposition and includes the preposition's object. It can act as a noun, adjective or adverb. Here are some examples of sentences with introductory prepositional phrases: "After work, I like to go out for dinner." "After work" is an adverb phrase telling when. "To Judy, he gave a gold bracelet." "To Judy" is a noun phrase and the indirect object of the verb. "Next door, the woman screamed." "Next door" is an adverb phrase telling where.
An appositive phrase is a noun phrase that describes another noun. Appositive phrases usually come after the nouns they describe, but they may come before it, as they do when they are introductory phrases. In the sentence, "A great composer, Wagner wrote famous operas," "a great composer" is an appositive phrase describing Wagner. Use introductory appositive phrases to give relevant information about the subject of a sentence, not irrelevant information. In the sentence "A great composer, Wagner liked to go to bed early," "a great composer" still describes Wagner, but it has nothing to do with the topic of the sentence: the time he liked to go to bed.
Participles are verbs ending in "-ing" or "-ed" that act as adjectives, and participial phrases are composed of participles and the other words that describe the participle's action. As introductory phrases, they usually modify the subject of the sentence. In the sentence, "Running like the wind, Sasha began to gain on him," "running like the wind" describes Sasha. Participle phrases are different from gerund phrases in that participles act as adjectives and gerunds act as nouns. Gerunds are not commonly used as introductory phrases, but participles are.
The word "to" followed by the base form of a verb is called an infinitive, and it is used to talk about the verb in the abstract, not connected with a subject. Introductory infinitive phrases act as adverbs. In the sentence, "To keep the dog, you must prove you can take care of it," "to keep the dog" is an infinitive phrase. Use introductory infinitive phrases to describe the purpose of the main verb in your sentence.
An absolute phrase includes a subject and information about it but not a verb. It has all the pieces of an independent clause except the verb "to be." In the sentence, "With the trees already growing back, the field was returning to nature," "with the trees already growing back" is an introductory absolute phrase. Absolute phrases can be placed anywhere in a sentence. Use them as introductory phrases to give more information about the circumstances in the main clause.