More or Less

You can change power or toughness. If you change a toughness of 1 to 0, the creature will probably die.

You can change the number of sides on a die. If you don’t have any five-sided, seven-sided dice, or d21s handy, you can use other dice to simulate the rolls.

You can change the number of dice rolled for an effect only if the original instruction included a number word to describe how many dice to roll. You can’t change the “a” in “roll a six-sided die,” but you could change the “two” in “roll two six-sided dice.”

You can change the numeral inside of a mana symbol, but you can’t modify the cost of a spell in time to pay less for it. It must already be on the stack to be a valid target for More or Less.

Indefinite articles such as “a” in “draw a card” aren’t number words and can’t be modified.

You can turn a 0 into a -1. Sources that would deal 0 or less damage don’t deal any damage, even in silver-bordered games.