Lay means “to put” or “to place.” It requires an object to complete its meaning. Principal forms are lay, laid, laid, laying.
Please lay the boxes there. I laid the message on the table.
Lie means “to recline, rest, or stay” or “to take a position of rest.” It refers to a person or thing as either assuming or being in a reclining position. This verb cannot take an object. Principal forms are lie, lay, lain, lying.
He’s been ill and lies in bed all day. The mail is lying on the secretary’s desk.
Hint: To determine whether to use lie or lay in a sentence, substitute the word place, placed, or placing (as appropriate) for the word in question. If the substitute fits, the corresponding form of lay is correct; if it doesn’t, use the appropriate form of lie.