Which of the following sentences contains an infinitive?
a. dave will take us to the field before the game.
b. heather wants to go to the store with us.
c. the car was parked next to the church.
d. let's walk to the picnic.

B Heather wants to go to the store with us.

An infinitive is a verb that hasn't been conjugated for a certain subject, so it sounds the same whether it's us, you, me, they, she or anyone doing it. (For example, while runs has an s in "she runs" and no s in "I run", with the infinitive, they are both the same: To run, as in "She wants to run" or :"I want to run")

Usually you can tell a verb is an infinitive if it has "to" in front of it.

In this sentence, hard is the adverb because it is modifying the verb.