For the life of me I can never get these two straight, so here is my quick reference:
Lay, Laid, Laying, Lays: to place, to set down, slang: to have sex
Lie, Lay, Lain, Lying, Lies: to be horizontal, to recline
Lie, Lied, Lying, Lies: to tell untruth
set, set, setting, sets: To put in a specified position; place
sit, sat, sitting, sits: (1) To rest with the torso vertical and the body supported on the buttocks. (2) To lie or rest: Dishes were sitting on a shelf.
Usage Note: Originally set meant “to cause (something) to sit,” so that it is now in most cases a transitive verb: She sets the book on the table. He sets the table. Sit is generally an intransitive verb: He sits at the table. There are some exceptions: The sun sets (not sits). A hen sets (or sits) on her eggs.