Bill Clinton is president. Therefore, dogs can fly. The first premise is true, but the second one isn't. But, IF both premises WERE true, then there is no possible way that the conclusion could be false. It is the FORM that guarantees this.
Deductive arguments draw out conclusions which are already contained in the premises.
Here is an analogy to help you to understand this point. Refrigerators preserve food. If the food that goes into the refrigerator is good, then the refrigerator maintains that goodness. If the food is already rotten when it goes into the refrigerator, then the food remains rotten. Likewise, the truth of reasons is preserved by a deductive argument form.