Sit and set are a lot like lie and lay: one is a reflexive verb (one you do only to/by yourself) and one is a transitive verb (that you do to someone or something else):
Sit means to assume a seated position, e.g., “He sits next to me in class.” The past tense is sat: “She sat in that chair last semester.” “He has sat through these lectures before.”
Set means to lay or place something, e.g., “Please set the vase on the table.” The past tense is set: “Bob set that glass down yesterday. He had set another one there last week.”
And remember, once you set something down, it sits there until it’s moved.