Notes:

“if A then B” means “if not B then not A” but doesn't mean “if B then A”.

e.g. a law-abiding citizen fulfills both A = you are underage and B = you order soft drinks, as opposed to alcohol
If you are underage, then you order soft drinks → if you order alcohol, then you are not underage BUT NOT if you order soft drinks, then you are underage, because you can be of age AND order soft drinks

the same logic applies

If I am having a good day, then I don't eat ice cream → If I eat ice cream, then I am having a bad day
follows formal logic
If I am having a bad day, then I eat ice cream → If I eat ice cream, then I am having a bad day
doesn't follet formal logic