And one more question, where can I find a list of terms that I can memorize for these formal logic conditions, (e.g. If/then, if and only if, if but only if, if yet only if, unless, not unless, etc.etc)

So If AND Only If, are basically two different formal logic statements squished together (bi conditional)? If that's true, then both must be either true/possible/able, or false/not true/not possible/unable. That I understand.

However, what's the contrapositive? is simply negating both terms the CP? For example as you say, if Hr If And Only If Gs, that means Hr (---)Gs. But is the CP /Hr (----)/Gs? and if that's the case, then does that mean that there are only two possible outcomes, or can one happen and one not happen?