Use the verb lie when the subject is doing the action.
I will lie down.
My dog likes to lie on the sofa.
Mom went to lie down.
Use the verb lay when the subject is doing an action to an object. The subject will lay the object down.
I will lay the placemats on the table.
Dad wants to lay some sod in the yard this weekend.
Mom went to lay the baby in the crib.
Use lie with a subject that is doing the action. Use lay when the subject must lay an object down.
It’s your turn. Decide if you should use lie or lay in this sentence. My cat will ___ on my bed all night. Did you figure out that the cat is the subject doing the action? Good for you.