In tonight's game, the knight went down to 0 hp. Later that round, the paladin granted the knight 7 temporary hp. As the DM, too lazy to slow the game down to look the precise rules up, I ruled that the knight returned to consciousness and was no longer dying. Was I right? How do temporary hp interact with the unconscious and dying states?
That's a good question. (And I agree with your off-the cuff ruling, as any decision made quickly is better than one that disrupts the flow of the game.)
Let's start with some definitions:
Not Real Hit Points: Temporary hit points aren’t real hit points. They’re a layer of insulation that attacks have to get through before they start doing damage to you. Don’t add temporary hit points to your current hit points (if your current hit points are 0, you still have 0 when you receive temporary hit points). Keep track of them as a separate pool of hit points.
Dying: When your hit points drop to 0 or fewer, you fall unconscious and are dying. Any additional damage you take continues to reduce your current hit point total until your character dies.
Regain Hit Points: When you are dying and receive healing, you go to 0 hit points and then regain hit points from the healing effect. If the healing effect requires you to spend a healing surge but you have none left, you are restored to 1 hit point.
Thus, because temporary hit points do not count as "healing" the character who was dying would remain at 0 hit points, unconscious, and should continue to make death saving throws. However the temporary hit points would protect against subsequent Coup De Graces and other DM agression.