After lunch yesterday she ____ down on the sofa for an hour.

Explication

Nothing follows the verb here — she goes down herself — so this is the intransitive verb lie — lay — lain, meaning to be in a horizontal position, and its past simple is lay. The look-alike verb lay — laid — laid always needs an object, so laid would demand something she put down, and lied comes from a third verb that shares the same base, lie — lied — lied, about telling untruths.