Lie vs. Lay:
These verbs are now synonyms, though some grammarians may disagree.
The big debate: According to the dictionary …
“Lie” means to be in a flat or horizontal position; or to be in a state of inactivity, concealment, or expectation (like “to lie in wait”). This verb is “intransitive,” or has no object: A person might lie down on a bed; a dog might lie outside on the grass; a book might lie on the shelf; a tiger might lie in ambush. When a person or thing is lying down, there is no other activity connected to the subject.
“Lay,” is the “transitive” one – or the one that takes a direct object and is comparable to the word “set” or “put”: “I always lay my keys on the table near the door;” “I laid the baby down in the crib;” “the cat laid his ears back, ready to pounce.” In the phrase, “to lay down the law,” the meaning is “to apply.”
WHY THE LANGUAGE CHANGE
See for yourself – and spot the confusion. Here are both words in base form, simple past tense, and past participle:
LIE LAY LAIN
LAY LAID LAID
In short, it's messy: the past tense of “lie” is the same as the base form of “lay.” To fuss over these verbs has proven too much for most people -- particularly when the meaning is clear whichever verb is used. Whether a lion is laying in a tree or lying in one, the mental image is the same.
It is now decades since I first started hearing “lay” instead of “lie,” and people who now use “lay” instead of the traditional “lie” are educated, intelligent people
-- so there is no social stigma. “Lie” and “lay” are either used interchangeably – or with a preference for “lay” – in movies and on TV, in books and magazines, digital and print, and in everyday conversation. Noted linguist John McWhorter validates the synonymous meaning of the two verbs himself in his book, “What Language Is” (2011). In effect, the distinction is now lost well enough so that both are correct and no cause for further debate.