An ‘infinitive’ is a verb form that is usually p[receded by the word ‘to’ and is used as a noun, an adjective, or an adverb. When you see the word ‘to’ before the base form of a verb, ‘to’ is not a preposition but part of the infinitive form of the verb.
Examples: ‘To volunteer’ is rewarding. [infinitive as subject]No one wants ‘to leave’. [infinitive as direct object]Their decision was ‘to merge’. [infinitive as predicate nominative]I felt the need ‘to call’. [infinitive as adjective]Everyone was prepared ‘to sacrifice’. [infinitive as adverb]
An ‘infinitive phrase’ contains an infinitive plus any complements and modifiers. Examples: Would you prefer ‘to sleep until noon’. ‘To speak slowly and clearly’ is important. We plan ‘to work safely and effectively’.
Occasionally, an infinitive phrase may have its own subject. Example: Our neighbor encourages ‘the dog to bark’. [Dog is the subject of the infinitive ‘to bark’. The entire infinitive phrase, ‘the dog to bark’ acts as the direct object of the sentence.]
Note that the subject of the infinitive phrase comes between the main verb and the infinitive. The subject of an infinitive phrase always follows an action verb,. Sometimes the word ‘to’ is dropped before an infinitive. Example: Let me [to]‘do the dishes’. We could have heard ‘a pin [to]drop’.