Lay and lie is pretty easy - Lay is a transitive verb, meaning it has an object. I lay the quilt on the bed. Lie is an intransitive verb, no object . I lie on the bed.
To clarify more, transitive verbs are followed by a noun. I bake the pie. I ride the bike. The same verbs can also be intransitive, I bake every morning. I ride to work most days.
Lie and Lay are mixed up by a lot of people.
I still say thongs and am probably the only person left who calls blue jeans, "dungarees". (almost 60 years old).
I was taught to remember there is "a rat" in separate. Works for me.