Complex sentence: Informal fallacies, when deductive, commonly occur in an invalid form, but by including an unstated co-premise, most deductive informal fallacies are actually valid, with the hidden co-premise false, making the argument unsound.
Split into simpler sentences: Informal fallacies, when deductive, commonly occur in an invalid form. By including an unstated co-premise, most deductive informal fallacies are actually valid, with the hidden co-premise false, making the argument unsound.