I used to think that the difference between "lay" (past tense) and "laid" (past tense) was that the former described an action that had already happened or it was ongoing, and that the latter described one that was happening right now.
And so ...
Mark hung his parka on the hook behind the door and laid on the bed.
(Mark is throwing himself on the bed, so laid should be used.)
Mark lay on the bed, thinking.
(Mark is already lying on the bed, so lay should be used.)
Is this correct or I'm wrong? Maybe "laid" should only be used when you are putting something down (not yourself)?