The First Thing to know is: there is no longer a difference between LAY and LIE.
“Lie”(or “lie down”) has traditionally been the word to describe putting yourself in a horizontal position either in bed, on the ground, or some other flat surface.
“Lay” has always been used to set or put something – an object — on a flat surface.
The similarity of these two words has led to verbal confusion among native English speakers for many years. Now, the word “lie” seems to be passing out of the language. Although it is still correct to use “lie down,” most people use “lay” for “lay down” and also its original use, “to lay (something) down.”
You can now use either word – lay or lie – in informal or formal situations and sound correct.
The Old Rule was, LIE is “intransitive” – that is, it does not take an object:
“He is going to LIE down for a nap.” (No object)
The new standard is, “He is going to LAY down for a nap.”
LAY has been, and still is, a “transitive verb,” meaning it takes an object:
“Please lay the plate on the table.” (“Plate” is the object.)
One reason for this change is, simply, confusion.
Look at the base form, past tense, and past participle of each verb:
LIE – LAY—LAIN vs LAY – LAID – LAID.
This is confusing!
This explains why native English speakers have generally stopped using “lie” and now use “lay” for everything.
Present tense: “I’m going to lay down.”
Past tense: “I laid down for a nap.”
Present Perfect: “I have laid awake.”
Many grammar sites on the internet are still explaining the old lie vs lay rules. But starting about 2010, in my listening experience, “lay” has gradually become the verb of choice.
LIE and LAY are examples of how language changes. English is not monitored by any governmental agency. Rather, words come and go according to those who speak the language.
English dictionaries include words that people use and remove words no longer in use. Language changes over time, and it is change that keeps a language alive.