Lie vs Lay
Quick answer: Lay needs an object — you lay something down. Lie doesn't — you lie down yourself. The trap: the past tense of lie is 'lay', which is why everyone mixes them up.
Lie
Verb (no object): to recline or rest flat. Past tense: lay; participle: lain.
“I need to lie down for a minute.”
Lay
Verb (takes an object): to put something down. Past tense: laid.
“Lay the book on the table.”
How to remember it
You LAY something down; people LIE down on their own.
Common mistake
'Go lay down' — no object there, so it's lie down.