Gettin' Laid While Lyin' Around

Lies are what you tell so you can lay someone on the side.

But more to the point, lay is transitive (i.e., it takes an object), whereas lie is not.

This is further complicated because some idiot decided the past tense of lie should be lay, which also happens to be a present-tense verb.

But back to how to know what to write. If you can lay something, it's lay: lay the book down, lay some bricks, lay your best friend's girlfriend. This use of lay is known as a transitive verb. It  transitions based on its meaning when combined with the object it affects.

Even Ratt knew this, of course:

With lie, there's no object: I'm going to lie down now, don't just lie there, he was lying on the bed. But note that in past tense, lie becomes lay: I lay there all day, NOT I laid there all day, BUT I laid (past tense of lay) the book down. In this usage, lie and lay are called intransitive verbs—they don't have objects, so they don't change.

Exceptions exist, mostly idiomatic: lay me down, I laid myself down, I lay down to sleep. Grammatically, myself and me are objects, so the verb changes from lie to lay. Similarly, it's lying low and lying around (no objects). But the flu can lay you low (you is an object, so the verb becomes lay).

So you can lay your partner, but don't just lay there (cuz laying there is wrong in so many different ways). Look for an object, and watch out for prepositions before objects, as they tend to indicate intransitive verbs—I lay WITH the pillows (intransitive) versus I laid THE pillows down (transitive).