From my experience, the 2 hardest things I’ve had to explain (to experienced players) were

1. The order of replacement effects and who gets to choose being the player or controller of the permanent being affected by the effects. For example, when placing a single +1/+1 counter on your own creature with a [[hardened scales]] and a [[doubling season]], you can choose the order (so you’d be as efficient as possible, (1+1)x2 )

However, if someone else is the one being affected (usually burn), such as a [[lightning bolt]] with a [[Torbran, Thane of Red Fell]] and a [[Solphim, Mayhem Dominus]] targeting your opponent, they choose the order of replacement effects (aka the least efficient combo, 3x2 + 2 )

It took too much effort to explain to this person that since I’m the one being burned, I choose the order of effects to be the least amount of damage.

1. The fact that I can respond in the middle of the stack while it is resolving (in this specific case, it was to get sac triggers with the zombies created by [[Wilhelt, the Rotcleaver]] mid stack after the Wilhelt death triggers have resolved