As you learned in the last lesson, "lie" means to be at rest, or to recline in a horizontal position. Its forms are: lie (present), lying (present/past progressive), lay (past), lain (participle). Examples: 1) Lie down. 2) Lying in the sun dries the skin. 3) The parcels lay on the table. 4) We have lain in the sun for thirty minutes.
Unlike "lie," "lay" is a transitive verb, so it always takes an object. Remember that "lie" never takes an object because it is intransitive. The forms of "lay" are lay, laying, laid, laid. Examples: 1) Lay the bricks here. 2) He was laying the bricks in rows. 3) Yesterday he laid the bricks ten high. 4) He has laid all the bricks in the wall.
Note that the present tense of "lay" is the same as the past tense of "lie."