How do you know what to negate here

So the answer was "E. A contemporary work of art that does not reflect the independent artistic conscience of the artist cannot be a work of true artistic excellence. However when I negated the sentence in my mind I changed Does not to does and cannot to can, I thought we are supposed to completely negate the sentence to be sure that we are strengthening with the necessary condition. My main question is why did we just change CANNOT to can and change (Does not), and how do we know which one to negate is there a step I'm missing or a key I am forgetting??