My physical therapist, a rather attractive young woman, was directing me to a table where she was going to hook me up to a neck stretching device that looked like it was left over from the Spanish Inquisition. "Lay down there!" she commanded.  

"Do you want me to lay there or lie there," I asked, hoping she would see the innuendo. No such luck. All I got was my neck stretched.

About 15 minutes later I was directed to another table and this time I was told to "lie down there."

 "Very good," I replied. Another physical therapist happened to overhear this exchange and we ended up discussing the proper usage. He commented that he always tells patients to "lie down,"but that one patient had corrected him, telling him he should tell people to "lay down." This patient also said that she was an English teacher. So, the explanation of why these two words are so misused finally surfaces. Even teachers don't know. 

It got me to thinking about the almost total misuse of the word lay in modern society. For example, I almost always hear people telling their pet dogs to "lay down," which is incorrect. About the only time I hear it used correctly is when someone says, "I got laid." And even that is questionable, or as the grammatically correct would say – a colloquial use.

For the lay person (pun intended) most explanations of the proper use of lay and lie don't help. Here is a good example from an academic writing tutor:  "Lie and lay are both verbs, but the difference is that 'lay' is a transitive verb and 'lie' is an intransitive verb." Now if you are like me, you either never knew, you don't remember, or you couldn't care less what it means to be a transitive or intransitive verb.

Without getting too much into conjugated forms of lay and lie, here is a simpler, but not simple, rule that may help.  Use lie when you are referring back to the subject of the sentence.

"You lie here." "Lie down." (for dogs and people) "I lie here every day." "Bob is lying over there." "Sue, lie over here." 

Use lay for everything else.  In effect, when you are putting something down, you are laying it down.

"Bob, if you are going to lay Sue, lay her over there where you normally lie."

Now at this point someone inevitably says to me,  "How about, 'Now I lay me down to sleep.'"

This is a case of putting something down (laying it down). It just happens that that something is you. Without the word 'me' in the sentence, it would correctly read: "Now I lie down to sleep."  You lie down, but you lay me down, regardless of who (or what) "me" might be.

I gave my physical therapist a quick helpful guide to remembering when to use lay and when to use lie.  "If you are doing it to someone else, you are laying them. If they are doing it to themselves, they are lying."  I don't think this helped her much, but I enjoyed saying it.