The verb sit is used when talking about a person or animal doing the action on their own.
I will sit on the sofa.
My students sit in their desks during class.
My dog likes to sit on the front porch.
In each of these sentences, the subject is doing the action on their own.
The verb set is used when the subject of a sentence places an object in a certain place.
Bob will set the cookies on the table.
Mary likes to set the volleyball to her teammates.
My dog set his bone on the floor by my feet.
In these sentences, the object was set, or placed, by someone else.
Use the verb sit when the subject of a sentence does the action themselves. Use the verb set when an object is being placed by someone else.
It’s your turn. Decide if you should use sit or set in this sentence. Don’t ___ the plate on that hot stove. Did you choose the verb set? Good for you!