lay, lie

Lay means put something down: you lay an object somewhere. Lie means recline or rest in a position: you lie down yourself. The object is a useful clue.

Put something down: Lay the blanket on the grass.

Recline: Lie down on the blanket.

The past tense is the tricky part

For put something down, the forms are lay, laid, and have laid. For recline, they are lie, lay, and have lain.

Put something down: Today I lay the blanket out. Yesterday I laid it out. I have laid it out before.

Recline: Today I lie in the sun. Yesterday I lay in the sun. I have lain there before.

The -ing forms are laying and lying. Lie can also mean say something untrue; that separate verb takes lied: She lied about the surprise.