I remember writing an article for the Surrey NOW in which I said, “She lays down on her bed.” I was writing in the present tense. Absolutely wrong. It’s supposed to be, “She lies down on her bed.”
A week later an English teacher wrote to me through the NOW. “You call yourself a teacher and you don’t know the distinction between lie and lay?”
According to Strunk and White, LAY is a transitive verb that should not be confused with the intransitive verb LIE. LAY takes an objective, LIE doesn’t. You always LAY something down.
There’s an old joke about the man who tells his dog to “LAY down.” The dog looks up at him and says, “It’s LIE down, stupid.”