This is a tough one (for me) to remember. It helps when I associate lay with objects and lie with people.
I will walk to the desk and lay the book down.
I am going to lie down for a nap.
The past tense of lay is laid-easy enough. Just when I think I’ve got it, it gets confusing again, because the past tense of lie is lay. (I get a headache just thinking about it.)
I walked to the desk and laid the book down.
I went to lay down for a nap.
This one is easier once you have the hang of past tense.
I had walked to the desk and laid the book down. (Same as past tense)
I had lain down for a nap.
I keep this chart in my little book of notes in case I forget.
PRESENT PAST PAST PARTICIPLE
Lie lay lain
lay laid laid