Is Sketch affected by accuracy drops or evasion?
Let's say an opponent used 6 Sand Attacks/Smokescreens on me and I then attempt to use Sketch; will Sketch be more likely to fail?
What if the opponent used Double Team or Minimize?
Does generation matter?
Sketch allows the user to learn the target's last move permanently, replacing Sketch in the process. It bypasses accuracy checks to always hit, unless the target is in the semi-invulnerable turn of a move such as Dig or Fly.
Bulbapedia has a good description of the move and everything you need to know, including when it can be used.