Sit means to take a seat. Set means to put something down. Sit does not take a direct object. Set does. Sit in the chair. Set the ticket on the desk. Same object test as lie versus lay.
The object test
Drop in a noun after the verb. If the sentence still needs that noun, set is right: Set the calculator down. If a person is just taking a seat, sit is right: Sit here. Past forms: I sat, I have sat. I set, I have set. Set does not change in the past.
Rise versus raise is the other object pair, in rise versus raise. Diction overall is in word choice. Grammar as a set is in ACT grammar rules.
The bottom line
No object: sit. Putting a thing somewhere: set.
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