These verbs cause trouble for many people. If you mean repose, use lie. If you mean set or put, use lay.
Can you help me figure out when to use the word lay instead of lie?
An easy way to remember which one to use is to recognize that lie does not take an object and lay does:
I lie down for a nap everyday.
The dog lies by the fire.
I lay the paper on the table.
I lay myself down (here myself becomes the object)
The past tense and the past participles of lie are lay and lain:
I lay down for a nap yesterday.
I have lain down every afternoon this week.
The past tense and the past participles of lay are laid and laid:
I laid the paper on the table yesterday.
I have laid it in the same spot for years.