Okay, here is a quick explanation:
lie, lay, lain (intransitive verb)
(to) lay, laid, laid (transitive verb)
He told her to lie down on the bed.
I lay in that position for twenty minutes.
That coat has lain there for a week.
Lay the papers on the floor.
Robert laid the book aside.
She had laid her car keys on the shelf.
Hope that helps.
Jeanne and everyone else who responded: Thanks!
The guard lie motionless on the floor
I've stepped into an alternate reality if anyone thinks the above sentence is correct.
Debbi, I agree.
Anyone who reads that is going to pause with his or her mouth wide open and think, "typo". Honestly, if you are talking about guards lying on the floor then it is probably more important to phrase the sentence in a way that is easy to read so as not to detract from the rising tension.
On a side note changing to a stronger verb could always help:
The guard was sprawled motionless upon the cold tile floor. (Sorry, I'm a bit addicted to adjectives)