This is the technical difference between lay and lie:
You lay an object onto a surface.
Again, you lay an object onto a surface. But a person/thing lies (itself) on the surface:
For a person, to lie + a preposition of place means to put yourself horizontally on a surface:
Now… to complicate matters – the past tense of lie is lay!
Present Continuous Past Past Participle
LAY LAYING LAID LAID
LIE LYING LAY LAIN
Also, native English speakers often say “I need to lay down” and “She’s laying on a towel on the beach.” – even though it’s incorrect!
We laid the flowers on the grave.
He’s just lying there on the couch watching TV.
There was an abandoned bicycle lying on the sidewalk.
This lesson refers to the meaning of “lie” as an object lying on a surface. There’s another meaning for “lie”: to say something that isn’t true. In this case, the past and past participle would be lied and lied: