Denying the Antecedent

You are committing a fallacy if you deny the antecedent of a conditional and then suppose that doing so is a sufficient reason for denying the consequent. This formal fallacy is often mistaken for modus tollens, a valid form of argument using the conditional. A conditional is an if-then statement; the if-part is the antecedent, and the then-part is the consequent.

Example:

If she were Brazilian, then she would know that Brazil's official language is Portuguese. She isn't Brazilian; she's from London. So, she surely doesn't know this about Brazil's language.