Use the verb sit when someone is doing it themselves.
I sit on the sofa.
The squirrel will sit in the tree.
Use the verb set when you are talking about someone placing an object.
I will set the book on the desk.
The baby set the toy on the shelf.
When talking to someone, you can invite them to sit themselves down.
Please sit down.
If they are holding something, you can invite them to set down the object.
Please set your coat on the chair.
Use sit when a person or animal is doing the sitting themselves. Use set when someone is placing an object.
▶It’s your turn. Choose either sit or set for the blank in this sentence. You can ___ the glass on the counter. Did you choose the word that goes with an object? Good for you!