If I’m understanding correctly to negate a conjunction : M -> N and O - we flip, negate + change the and to or leaving us with : / N or / O -> /M

To negate a disjunction: /N or /O -> /M

we flip, negate + change or to and

leaving us with : M -> N and O

It helped me to think of it as we cannot go backwards once we have the contrapositive chain.

Cat -> Mammal -> Animal

=

/Animal -> /Mammal -> /Cat

we can only go forward not backwards. we can conclude that if Tom is not a Mammal he is not a Cat. But we cannot reverse the chain /Mammal tells us nothing about Tom in relation to Animals. (i hope this helps someone! we’ve all got this :))