When I did a show about lay versus lie many months ago, listeners wrote in asking me to do a follow up show on sit versus set because the problem with sit and set is similar to the problem with lay and lie -- so here it is!
Transitive Verbs
Like lay, the verb set requires an object. You set something, the object, down. For example, I set the book on the desk. Or if you want to get more abstract, you can set a date. There is some object that receives the action of the verb. In my examples, it is the book and the date. On the other hand, like lie, sit doesn't require a direct object; it's something you do. I sit on the sofa.
Verbs, such as set, which require an object are called “transitive verbs,” and verbs, such as sit, which don't require an object are called “intransitive verbs.” The way I remember the difference is to think of transitive verbs as transferring their action to an object.
Sit Versus Set
I also have a good memory trick to help you remember the difference between sit and set. When you're training a dog, you tell her to sit. My first dog's name was Dude and she was a girl, so we would tell her, “Sit, Dude. Sit.” And she would plop her little bottom down. She was a good dog. She was a bull mastiff, so actually her bottom wasn't that little.
So get that image in your mind of a big bull mastiff responding to the command “Sit.” That is how you use sit – for the action of sitting.
Set, on the other hand, requires an object. I would set Dude's leash on the table, but she would still think we were going for a walk. I know she saw me set it down, but she was always full of hope. In those examples, the leash and the word it were the objects. I set the leash on the table, and she saw me set it down.
So remember that a dog (or person) sits, and you set things like leashes down.