lie (v.) to rest or recline example: I need to lie down. example: The books are lying on the floor.
The principal parts for lie are: lie, lay, lying, lain.
lay (v.) to put down, to placeThe principal parts for lay are lay, laid and laying.example: He will lay himself on a bed of nails. example: He laid the book on the floor.
Ask yourself, “Lay what?” In the sentences above, the answer would be the book, and himself. If there is no answer, use lie. Strategy number two: If you can substitute put, use lay. Otherwise use lie.