What happens to a creature that has 2 +1/+1 counters when a -1/-1 counter is added?

When a creature has two +1/+1 counters and one -1/-1 counter is added, the interaction between these counters leads to a net result of one +1/+1 counter remaining on the creature.

Counters in Magic: The Gathering are applied in a way that their effects can cancel each other out. In this case, the creature starts with two +1/+1 counters, which increase its toughness and power count. When a -1/-1 counter is added, it effectively decreases the creature’s toughness and power by one. Since there are two +1/+1 counters, the addition of one -1/-1 counter will effectively neutralize one of the +1/+1 counters.

Thus, after the application of the counters, one +1/+1 counter remains on the creature. This process is not about the creature being destroyed or the simultaneous removal of both counters; it’s a matter of the counters offsetting each other. Therefore, the creature ends up with one +1/+1 counter after resolving the effects of both the +1/+1 and -1/-1 counters.