Lie means to recline. Lay means to put something down. Lie does not take a direct object. Lay does. Lie down. Lay the calculator on the desk. Diction pairs like this are in word choice on ACT English.
Present, then past
Today I lie down. Yesterday I lay down. I have lain down. Today I lay the book here. Yesterday I laid the book here. I have laid the book here. The overlap that confuses people is that the past of lie is lay.
Sit versus set is the same object test: sit is recline-adjacent, set takes an object. Idioms that are not this pair are in ACT idioms. Grammar overall is in ACT grammar rules. A nearby mix-up, like versus as, is in like versus as.
The bottom line
If you can put a thing after the verb, you want lay. If a person is reclining, you want lie.
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