Denying the Antecedent is where the conclusion of two statements is thought to falsify the initial truth statement. For this to happen we need an initial statement of truth. Then we make a second statement about the initial statement that is also true. From this if we conclude that the initial statement is false we have denied the antecedent, thus committing the fallacy.
Denying the Antecedent Example
If Mike is a Muslim then he is also a human being.
Mike is not a Muslim
Therefore Mike is not a human being.
If A then B
A is not true
Therefore B is not true