There are rules for how to know when each of these things get "triggered" by the actions of the other players or NPCs. And, let me tell you now that the secret to succeeding in this game is knowing your tactics cold and knowing your "place" in the "tactics chain triggers".
This is what I liked about early versions of D&D, combat was very abstract so being a tactical wizard wasn't very important to character success as a player, nor was it required to run an enjoyable by combat-loving players combat as a DM. This was very good from my POV as I have ALWAYS been horrible at tactics. I'm pretty good at strategy (heck, I was winning with Japan in a War in the Pacific boardgame long after Japan should have been losing) but at tactics, I literally suck. I suspect I would not be a popular player in modern version of D&D as not only am I not really interested in hour-long combats, but I suck at tactics.
They have been given a "shifting" power that allows them to effectively move out of the way when attacked. We argued about that all night on or off each time we played. The rules were not clear enough for us to settle it conclusively and it caused some aggravation around the table.
This is anther thing that bothers me about rules-heavy games like modern D&D, such problems don't seem easy to solve by a few minutes of discussion and a DM decision on how the rule will work in that DM's campaign. That's how we used to solve them but that doesn't seem to happen as often any more. Probably because the rules have become the game. The rules aren't what you use the frame the adventures (which are the real game), the rules and player mastery of them often seem to be the core of the modern rules heavy game (and adventures exist so player can show their rules mastery).
