Several features like the Lucky feat, the divination school, and bardic inspiration can change the outcome of a roll by adding or changing the result.
The lucky feat states:
You can choose to spend one of your luck points after you roll the die, but before the outcome is determined.
The divination school states:
You can replace any attack roll, saving throw, or ability check made by you or a creature that you can see with one of these foretelling rolls. You must choose to do so before the roll, and you can replace a roll in this way only once per turn.
And the bardic inspiration states:
The creature can wait until after it rolls the d20 before deciding to use the Bardic Inspiration die, but must decide before the DM says whether the roll succeeds ar fails.
My issue is understanding the concept "Before the outcome is determined". Some examples:
Bob rolls a 1 on an attack roll, the outcome is automatically determined, he failed. He can't roll a luck dice, because 1 is a natural failure for an attack.
Tim the ogre attacks Bob, and rolls a 20 on the dice. It's a critical and automatic succes. Bob can't spend a luck dice to change Tim's roll, because a 20 is a natural succes.
Bardic inspiration is out of the formula, because modifiers don't change the outcome of an automatic succes or failure.
In the case of the divination school Bob the Wizard can expend those dice when the GM says "the ogre attacks you", Or when Bob decides to attack and choose a foretelling dice.
My question is. Did I get everything right, or I missed something? And, What happens with the divination school feature when you roll behind the DM screen?
Thanks for your answers, and sorry for my english, It isn't my native language.