To Lay down or to Lie down （Kang Liao）
We hear people say, "I'll lay down" or "I'll lie down." Which is right? These two irregular verbs are very difficult to use correctly, even for native speakers of English, because the past tense of "lie" is "lay," which has helped "to lay down," originally a mistake, become an expression that has been widely accepted in colloquial American English, just like the idiom “to lay low.”
"Lay" (laid, laid, laying) means to "put, place” or "prepare" and is usually a transitive verb. As such, it takes an object; for instance, to lay a baby in the crib, to lay eggs, to lay an ear to the door, to lay the table for dinner, to lay the blame on somebody, to lay a plan, to lay a penalty on the offender, to lay a case before the grand jury, or to lay claim to the estate. See The American Heritage Dictionary for more examples.
"Lie" (lay, lain, lying) means to "recline" or "be situated." It is an intransitive verb, which means it does not take an object; for instance, She often lies down after lunch. The garbage has lain there for two weeks. His motive has been lying hidden. The ocean lies to the west of the mountain. Remember that in to lie down, “down” is an adverbial, not an object. As if this is not complicated enough, the forms of this verb can be easily confused with the regular form (lied) when "lie" means to "give a false statement deliberately."
Perhaps the popular song "A Bridge over Troubled Water" by Simon and Garfunkel can help us remember the meaning of "lay" and that it is a transitive verb. In the refrain "Like a bridge over troubled water, I will lay me down" the verb "lay" means to place, and "me" is the object. "To lay me down" here suggests placing himself down in the water metaphorically for a person to walk over. That is not as comfortable as to "lie down."