Damage in Magic: The Gathering resets at the end of each turn during the cleanup step.
No, Nekusar does not deal commander damage in Magic: The Gathering.
No, infect does not count as combat damage in Magic: The Gathering.
Yes, in Magic: The Gathering, loss of life is considered a form of damage.
No, destroying a permanent does not count as dealing damage in Magic: The Gathering.
No, a copy of a commander does not deal commander damage in a game of Magic: The Gathering.
Yes, double strike triggers combat damage twice in Magic: The Gathering.
No, the destroy effect in Magic: The Gathering does not deal damage to the target. It simply removes the target from the game.
No, copies of commanders do not deal commander damage in Magic: The Gathering. Only the original commander card can deal commander damage.
No, partner commanders do not share commander damage in Magic: The Gathering. Each partner commander tracks their own commander damage separately.
In Magic: The Gathering, excess damage dealt to a player is ignored, but excess damage dealt to a creature is removed at the end of the turn.
The official ruling on Phyrexian Obliterator in Magic: The Gathering is that when it is dealt damage, that damage must be paid in the form of permanents being sacrificed.
In Magic: The Gathering, lifelink allows a creature to gain life equal to the damage it deals when attacking.