Lie and lay are probably the most troublesome and wrongly used words I see regularly (or hear in conversation).
As you probably heard in school, lie means to recline; lay means to put.
Here are two simple rules.
1. If you will remember the principal parts of the verb (present, past, and past participle), you can’t go wrong. They are lie, lay, and have lain.
2. No form of the verb lie is followed by a direct object (noun or pronoun). That is, you can’t lie a book down.
Lay means to put or set down. The principal parts are lay, lay, and laid. Usually an object follows a form of the verb lay. (He laid the box on the floor.)
The problem for many is that the past tense of lie is lay. Some writers can’t remember that and write, “He laid down to rest.” They mean he was put down to rest. Does that mean they killed him?
Somewhere I picked up this tip. Memorize one simple sentence where you use lie and lay. Try this: Hens lie down to lay eggs. A friend learned by repeating this simple statement to herself: You lay something down, and people lie down by themselves.