"if not a, then not b" is equivalent to "if b then a" and is neither equivalent to nor the opposite of "a implies b". I'm having trouble understanding your point under these confusions. Is it that being able to explain is a sufficient but not a necessary condition for understanding?
Point taken. My point is just because I can simply explain it does not imply clear understanding. And not being able to explain isn't a reliable indicator of lack of understanding
I understand the point of your comment. Just the last sentence ("Not being able to...") would have been enough. But your monkey example is confusing, because "=>" usually means "implies", not "because", and with that meaning there is nothing weird about the last statement.
A statement like "if not a, then not b" is not applicable unless a "implies" b and a "is because of" b.
Correct me if I got it all wrong somehow.