I'm afraid I have to join the others on this one. This is has got to be the hardest concept I have encountered so far. I'm glad that you mentioned that if you use "nunca" or "siempre" then the verb would be in the simple past. However, that is where my understanding stops. I tried thinking about whether one is recent past (perfecto) and one is more distant but still somewhat recent (indefinido), but that is confusing and probably not correct. Thank you also for stating that this is something that is different across all the different countries. I will just keep trying and hope that something sticks at some point.