Which words in the sentence make up the infinitive phrase?
after you go with your friends to the movie, will you promise to come back soon?
a. after you go
b. to come back soon
c. will you promise to
d. to the movie

Infinitive is a verbal made up of the word "to" plus a verb, still with a prepositional phrase starting with the word "to" which make up a "to" plus a noun or pronoun and any modifier. Some examples of infinitives are to eat, to climb, to dance, to wash and a lot more. Furthermore, an infinitive phrase is a group of word with and infinitive or infinitives plus a modifier or modifiers, a noun, pronoun, noun phrases that serves as the actors, direct or indirect object or objects. In the sentence above the infinitive phrase is b. to come back soon.