(verb): to act in response to something

re (back) + act (do)

How did he react when he heard the news?

(noun): something which acts or acts upon something else

ag (do, act) + ent (n. act)

The travel agent helped her purchase the tickets.

(adjective): involving movement, moving about

act (do, act) + ive (adj. having the quality)

Joan is an active child.

(verb): to excite, to disturb, to stir up

agit (do, act) + ate (v. cause to be)

The washing machine agitates the load of laundry.

(adjective): can be heard

aud (hear) + ible (adj. ability)

Your voice is barely audible over the sound of the train whistle.

(noun): a place where one goes to hear something

audit (hear) + orium (n. the place)

The band played in the school auditorium.

(adjective): related to the sense of hearing or the ear

aur (to hear) + ic (adj. quality) + 

ul  + ar (adj. related to)

The auricular ability of the cat is well-known.

(adjective): loving

amor (love, like) + ous (adj. having the quality)

The actor played an amorous role.

(adjective) friendly, agreeable

ami (love, like) + able (adj. worth, ability)

He seems to be an amiable man.

(adjective): friendly

amic (love, like) + able (adj. worth, ability)

The two neighbors came to an amicable decision about the fence.

(noun): a living creature

anim (mind, life, spirit, anger) + al (n. action, result of action)

A dog is an animal.

(verb): to give spirit or support, to supply movement

anim (mind, life, spirit, anger) + ate (v. cause to be)

The artist animated the cartoon.