The verb lie is used with the subject of the sentence. The subject can lie down.
I will lie on the couch and read.
I wish the dog would lie down and stop running around.
The baby needs to lie down and take a nap.
The subject always uses the verb lie.
Use the verb lay when talking about an object. A subject of the sentence will lay down an object.
Dad will lay some drop clothes over the wooden floor before he paints.
Mom will lay my baby brother down for a nap.
My sister wants to lay some placemats on the table before we eat.
A subject will always lay down an object in a sentence.
Use the verb lie when the subject itself is doing the action. Use the verb lay when talking about laying down an object.
▶It’s your turn. Choose lie or lay for this sentence: The gardener will ___ some sod in our front yard. Did you figure it out? Good for you!