Affirming the Consequent, Denying the Antecedent

Both of these are misnomers. When you "affirm the consequent", you are actually affirming the antecedent upon observing the consequent.

If P, then Q.
Q.
Then P.

This is the fallacy that they call affirming the consequent. The fact that its misnamed makes it more difficult to remember.

Similarly, denying the antecedent:

If P, then Q.
Not P.
Then not Q.

Another fallacy, but again, it is misnamed. The denial of the antecedent is observational. The fallacy lies in denying the consequent based on the observation of not P.