An infinitive is the simple present form of a verb used as either a noun, adjective, or adverb. The verb of the infinitive is normally preceded by the word to. When the infinitive follows some verbs as the direct object, the to may be dropped.
An infinitive phrase is the infinitive plus any complements and any modifiers of the infinitive and complements.
As a Noun: He helped to write the program.In the above examples, the infinitive is italicized and the infinitive phrase is underlined.
As an Adjective: Lydia was looking for a way to earn money.
As an Adverb: He shouted to get our attention.
To Dropped: He helped write the program.