The transitive verb set means "to put" or "to place"; it takes a direct object, and its principal forms are set, set, and set. The intransitive verb sit means "to be seated"; it does not take a direct object, and its principal forms are sit, sat, and sat.
Examples:
Last night I set the table for dinner. When I sat down for dinner last night, only my cat joined me.
Practice:
(a) If a man knew anything, he would _____ in a corner and be modest.
(b) Have you _____ any goals for the new year?
