There are some verbs in English that some people find confusing.
And I think near the top of that list is "lie/lay."
So let's outline the problem.
We have one verb that is intransitive--that would be "lie."
So: "Today I lie down; yesterday I lay down."
Then we have "lay," which is transitive.
In other words, it can take an object.
So we say, "Today, I lay the book down; yesterday I laid the book down."
What's so confusing about these two is that the past tense of one ("lie/lay") is the present
tense of the other, ("lay/laid").
How did that happen?
Well, that happened because these two verbs are actually historically related to each
other.
If we go all the way back to Old English, we see that there are two kinds of verbs.
There's one kind of verb that forms the past tense by changing the vowel.
So: "ring/rang," "swim/swam," "lie/lay."
And we still have some of those in English.
The other kind of verb in Old English takes a "duh" sound, a "D," to form the past tense.
So: "talk/talked," "lay/laid."
In Old English what would sometimes happen with an intransitive verb like "lie," is that
you could create an "-ed" verb to make it transitive.
So we took "lay" and created the transitive verb.
So when in doubt, just remember that if there's an object, it will be "lay/laid."
And if there's no object, and you just want to take a nap, it's "lie/lay."