If the copied creature has in its mana cost, X is considered to be 0.

If Cursed Mirror somehow enters the battlefield at the same time as a creature, it can't become a copy of that creature. You may choose only a creature that's already on the battlefield.

While it is copying a creature, Cursed Mirror does not have ": Add " (unless the creature it's copying also has that ability).

If the copied creature is a token, Cursed Mirror copies the original characteristics of that token as stated by the effect that created the token. Copying a token doesn't make Cursed Mirror a token. Similarly, if Cursed Mirror itself is a token, copying a nontoken permanent doesn't make it stop being a token.

If something else becomes a copy of Cursed Mirror while Cursed Mirror is a copy of a creature, the duration of the effect is not copied. The new copy will still be a creature (with haste) after Cursed Mirror stops being a copy at the end of the turn.

Any enters-the-battlefield abilities of the copied creature will trigger when Cursed Mirror enters the battlefield. Any "As [this creature] enters the battlefield" or "[This creature] enters the battlefield with" abilities of the copied creature will also work.

If the copied creature is copying something else, then Cursed Mirror enters the battlefield as a copy of whatever that creature is copying.

Cursed Mirror copies exactly what was printed on the original creature (unless that creature is copying something else or is a token; see below). It doesn't copy whether that creature is tapped or untapped, whether it has any counters on it or any Auras or Equipment attached to it, or any non-copy effects that have changed its types, color, power and toughness, and so on.