lay / lie / laid / lain
To put something down versus to recline.
Lay takes an object: lay the key on the desk. Past tense and past participle: laid (She laid the key down. She has laid the key down.). Lie means recline and takes no object: lie down. Past tense: lay (She lay down). Past participle after have: lain (She has lain down). The common slip is lay down for lie down.
Examples
UseLay the key on the desk. (Takes an object.)
UseI will lie down. (No object.)
UseShe lay down an hour ago. (Past of lie.)
UseShe has lain down. (Past participle of lie.)
AvoidI will lay down. (If you mean recline.)