lay v. lie

Lay means ‘to put down’. Lie means ‘to recline’. Objects are laid down; people lie down by themselves. Lay is a transitive verb, taking a direct object: ‘Kenneth laid the pen on the desk.’ Here, ‘pen’ is the object. Lie is an intransitive verb, so it has no direct object: ‘Sally lies down’ (despite what Eric Clapton thinks).

The situation is further muddled because the past tense of ‘lie’ is ‘lay’: ‘Yesterday, Sally lay down on the divan.’ The past participle of ‘lie’ is ‘lain’. Both the past tense and the past participle of ‘lay’ is ‘laid’.

Grammar Girl on lay v. lie