Lie (to fib): lie, lying, lied, lied.
Lay (to set down): lay, laying, laid, laid.
Lie (to recline): lie, lying, lay, lain.
The first (to fib) is a simple verb. The second (to set down) is a transitive verb—it conveys an action from subject to direct object. The third (to recline) is another simple verb.