In the strictest grammatical sense, 'lie' is
an intransitive verb, whereas 'lay' is transitive. Conceptually, 'lie' is
something you do to yourself, and 'lay' is something you do to something/someone
You look tired. Lie down and rest.
Lay down the keyboard and step away from the
The past tense,
however, gives many people fits. The past tense of 'lie' is 'lay', and the past
tense of 'lay' is 'laid'.