I have a creature with 3/2 but my opponent use an enchantment
a) which make it -3/0, can i attack now with 0/2 creature?
b) which make it -5/0 what happens when i attack with -2/2 creature?
In either case, you can still attack. However, a creature with 0 or less power deals no combat damage, so in many scenarios attacking won't do much.
Swamp Mosquito, Signal Pest, and Xantid Swarm are all 0-power creatures that have abilities that only do something when they attack. They'd be especially useless if they needed a power boost to even be able to do that.
Note that creatures with Defender *can't* attack (and many of those also have 0 power). Guardians of Meletis can't attack - not because it has 0 power, but because it has Defender.
Even Char-Rumbler can attack...
When someone has an imaginary friend, we call it mental sickness. When a bunch of people have the same imaginary friend, we call it a religion.
What happens when a creature with -1 power attacks ? Does that damage harm to you whereas opponent ?
If a creature has a power of 0 or less, it doesn't deal any damage. So if a creature with a power of -3 attacks and goes unblocked, the defending player will not take any damage (your opponent does not gain life, and you do not lose life).
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