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-translating.