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

Use Lay the key on the desk. (Takes an object.)

Use I will lie down. (No object.)

Use She lay down an hour ago. (Past of lie.)

Use She has lain down. (Past participle of lie.)

Avoid I will lay down. (If you mean recline.)