If the resolution of a triggered ability involves casting a spell, that spell can't be cast if a spell with split second is on the stack.

After a spell with split second resolves (or otherwise leaves the stack), players may again cast spells and activate abilities before the next object on the stack resolves.

In a Commander game, combat damage you're dealt by a commander is still tracked, even if it doesn't change your life total.

Angel's Grace doesn't prevent damage. It only changes the result of damage dealt to you. For example, a 5/5 creature with lifelink that deals damage to you will still cause its controller to gain 5 life, even if that damage reduces your life total from 3 to 1.

If you have less than 1 life, damage dealt to you reduces your life total further below 0 (as normal).

Casting a spell with split second won't affect spells and abilities that are already on the stack.

You can't pay more life than you have, even if you won't lose the game.

Players may turn face-down creatures face up while a spell with split second is on the stack.

Players still get priority while a card with split second is on the stack; their options are just limited to mana abilities and certain special actions.

Angel's Grace doesn't stop loss of life from effects that say that you lose life.

Split second doesn't stop triggered abilities from triggering, such as that of Chalice of the Void. If one does, its controller puts it on the stack and chooses targets for it, if any. Those abilities will resolve as normal.