A sentence must have a subject and a verb. That may be all it has; "I see" is a complete sentence. Most sentences have additional elements, and some have many more, but the subject-verb relationship forms the core. It is possible to have extra verbs -- actions that the sentence's subject does not perform. Instead, these verbs act as nouns, adjectives and adverbs. The umbrella term for them is "verbals." You find verbals in gerund phrases, participial phrases and infinitive phrases.
A gerund phrase always uses a present participle. The present participle is the form of a verb that ends in -ing. A gerund phrase acts as a noun. It performs the functions typical of one, often serving as a subject, object or direct object. In the sentence "Seeing the menu made me hungry," the main verb is "made." "Seeing the menu," then, is the subject. The three words act together as a noun. You could replace them with "it" and the sentence would still work: "It made me hungry." Try that with the actual verb, and the result is nonsense: "Seeing the menu it me hungry."
Present Participial Phrase
Participial phrases use either the present participle or past participle of a verb. The present participle is the -ing form, the same one used in gerund phrases. The difference between such a participial phrase and a gerund phrase are their grammatical functions. The participial functions as an adjective, not a noun. In the sentence "The woman reading the menu appears hungry," the subject is "the woman." It is the woman who performs the action -- who appears. The phrase "reading the menu" simply describes her.
Past Participial Phrase
A phrase that uses a past participle also functions as an adjective: In the sentence "Menus studied on empty stomachs are always impressive," the main verb is "are." "Menus" is the subject, which the phrase "studied on empty stomachs" describes. When in doubt, determine which verb is the main verb and which verb is a verbal by experimenting with omission. Compare "Menus are always impressive" to "Menus studied on empty stomachs." The first is a complete sentence. The second is incomplete, so you know "are" is the main verb.
An infinitive verb form is sometimes called the dictionary form of the verb. It is not conjugated and begins with "to." Infinitive phrases are the most flexible. They can function as nouns. In the expression "To err is human," the infinitive is the subject. In "I hate to interrupt," the infinitive is the object. They can also function as adjectives, as in "She is not a person to trust." In a sentence like "They called her to tell her the news," the infinitive functions as an adverb. It modifies the main verb, "called."
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