Question

When is work negative? (When dealing with work done by an electric field on a charged particle)

Answer 1

Usually it just means reverse direction.

Answer 2

reverse direction with respect to?

Answer 3

if the electric field is positive and the particle is negative, then the opposite direction is repulsion. OR if the electric field is positive and so is the particle then the reverse direction is attraction.
if the field does work on the particle, but the particle moves away from the direction of the force, the work done is negative.