Cannot is another way of saying "not," which is a Group 4 (negate necessary) condition.
You're each saying one of the contrapositives of the statement, so you're both correct.
Another thing that may help is to translate a rule, then immediately check the translation against the original English. Make certain they match, then move on to the next rule and repeat.
That's what I did when I used to have issues with mis-trans…