The verb sit is used with the subject of the sentence. The subject of a sentence can sit.
Bob will sit on the sofa.
You can sit anywhere you like.
The bird sits on my deck railing every morning.
The verb set is used with the object of a sentence. The subject of the sentence will set an object down.
You can set the groceries on the counter.
I set your keys next to your bag.
We will set the chairs on the patio.
The subject of a sentence can sit. The subject of a sentence will set an object. We ourselves can sit, but we set an object.
▶It’s your turn. Would you use sit or set in this sentence? The baby will ____ on my lap. The baby is the subject doing the action. Did you choose the verb sit? Good for you.